Providing spatial displacement of transmit and receive modes in lidar system

ABSTRACT

A light detection and ranging (LIDAR) system includes a laser, a transceiver, and one or more optics. The laser source is configured to generate a beam. The transceiver is configured to transmit the beam as a transmit signal through a transmission waveguide and to receive a return signal reflected by an object through a receiving waveguide. The one or more optics are external to the transceiver and configured to optically change a distance between the transmit signal and the return signal by displacing one of the transmit signal or the return signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 16/783,550, filed Feb. 6, 2020, which claims the benefit of and priority to U.S. Provisional Patent Application No. 62/837,050, filed Apr. 22, 2019. The entire disclosure of U.S. patent application Ser. No. 16/783,550 and U.S. Provisional Patent Application No. 62/837,050 are incorporated herein by reference in its entirety.

BACKGROUND

Optical detection of range using lasers, often referenced by a mnemonic, LIDAR, for light detection and ranging, also sometimes called laser RADAR, is used for a variety of applications, from altimetry, to imaging, to collision avoidance. LIDAR provides finer scale range resolution with smaller beam sizes than conventional microwave ranging systems, such as radio-wave detection and ranging (RADAR). Optical detection of range can be accomplished with several different techniques, including direct ranging based on round trip travel time of an optical pulse to an object, and chirped detection based on a frequency difference between a transmitted chirped optical signal and a returned signal scattered from an object, and phase-encoded detection based on a sequence of single frequency phase changes that are distinguishable from natural signals.

SUMMARY

Implementations of the present disclosure are generally related to a system and method for providing optical detection of range using LIDAR, including but not limited to a system and method for providing spatial displacement of transmit and receive modes in coherent LIDAR.

In a first set of implementations, an apparatus includes a transceiver and a free space optic. The transceiver may be configured to transmit a first signal from a laser source along a transmission path and receive a second signal reflected by a target. The free space optic may be configured to direct the second signal along a receiving path such that the second signal is not received at the transmission path. The transceiver may be a monostatic transceiver including a single-mode waveguide. The free space optic may be positioned between the laser source and the single-mode waveguide. In some implementations, the free space optic is one of an optical circulator, an optical isolator, a polarization beam splitter/combiner, or a fiber coupled optical circulator. In some implementations, the free space optic is one of an optical isolator or a polarization beam splitter/combiner. The free space optic may be configured to polarize the first signal from the transmission path with a first polarization, and polarize the second signal with a second polarization that is orthogonal to the first polarization. In some implementations, the free space optic may be a fiber coupled optical circulator including a first port connected to the transmission path, a second port connected to a tip of the single-mode waveguide, and a third port connected to the receiving path. The free space optic may be configured to permit a signal to travel in a direction from the first port to the second port and do not permit a signal to travel in a direction from the second port to the first port. The free space optic may be configured to permit a signal to travel in a direction from the second port to the third port and do not permit a signal to travel in a direction from the third port to the second port. In some implementations, the apparatus may further include a collimation optic positioned between the single-mode waveguide and the target. The collimation optic may be configured to collimate the first signal transmitted from the single-mode waveguide, and focus the second signal into a tip of the single-mode waveguide.

In a second set of implementations, an apparatus includes a transceiver and a free space optic. The transceiver may be configured to transmit a first signal from a laser source in a transmission mode and to receive a second signal reflected by a target in a receive mode. The free space optic may be configured to spatially separate the transmission mode and the receive mode by optically changing a distance between the first signal and the second signal. In some implementations, the transceiver may be a bistatic transceiver comprising a transmission waveguide and a receiving waveguide. The free space optic may be positioned between the bistatic transceiver and the target. In some implementations, the free space optic may be a birefringent displacer configured to displace one of the first signal and the second signal in a direction orthogonal to a direction of the one of the first signal and the second signal. In some implementations, the apparatus may further include a collimation optic configured to shape the first signal transmitted from the transmission waveguide and to shape the second signal reflected by the target. In some implementations, the apparatus may further include a polarization transforming optic configured to adjust a relative phase between orthogonal field components of the first signal and the second signal. The birefringent displacer and the polarization transforming optic may be positioned between the bistatic transceiver and the collimation optic. In some implementations, the receiving waveguide may be spaced apart from the transmission waveguide by a separation. The birefringent displacer may be configured to displace the one of the first signal and second signal by a distance orthogonal to the direction of the one of the first signal and second signal. The distance may be based on the separation. In some implementations, the birefringent displacer may have a dimension along a longitudinal axis of the apparatus, wherein the dimension is sized such that the distance is about equal to the separation. In some implementations, the birefringent displacer may be configured to cause the first signal not to be displaced and cause the second signal to be displaced based on the separation such that the displaced second signal is incident on the receiving waveguide.

In a third set of implementations, a method includes transmitting, from a laser source, a first signal from a transmission waveguide of a transceiver. The method may include receiving, by a receiving waveguide of the transceiver, a second signal reflected by a target. The method may include displacing, by a birefringent displacer, one of the first signal and the second signal in a direction orthogonal to a direction of the one of the first signal and the second signal. The birefringent displacer may be positioned between the transceiver and the target. The receiving waveguide may be spaced apart from the transmission waveguide by a separation. The displacing may include displacing of the one of the first signal and the second signal by a distance orthogonal to the direction of the one of the first signal and second signal, wherein the distance is based on the separation. The receiving waveguide may be spaced apart from the transmission waveguide by a separation. The displacing may include not displacing the first signal in the direction orthogonal to the direction of the first signal, and displacing, by the birefringent displacer, the second signal in the direction orthogonal to the direction of the second signal based on the separation so that the displaced second signal is incident on the receiving waveguide. In some implementations, the method may include adjusting, by a polarization transforming optic, a relative phase between orthogonal field components of the first signal and the second signal. The method may include shaping, by a collimation optic, the first signal transmitted from the transmission waveguide and the second signal reflected from the target. The birefringent displacer and the polarization transforming optic may be positioned between the transceiver and the collimation optic. In some implementations, the adjusting step may include adjusting, by the polarization transforming optic, a first linear polarization of the first signal into a first circular polarization in a first direction, and adjusting, by the polarization transforming optic, a second circular polarization of the second signal reflected by the target into a second linear polarization orthogonal to the first linear polarization and wherein the second circular polarization has a second direction opposite to the first direction. In some implementations, wherein the displacing may include displacing, by a first birefringent displacer of the birefringent displacer, a first component of the first signal based on a first linear polarization of the first component and not displacing a second component of the first signal based on a second linear polarization of the second component orthogonal to the first linear polarization. The adjusting may include rotating, by the polarization transforming optic, the first linear polarization of the first component to the second linear polarization and rotating the second linear polarization of the second component to the first linear polarization. The displacing may include displacing, by a second birefringent displacer of the birefringent displacer, the second component of the first signal incident from the polarization transforming optic based on the first linear polarization of the second component and not displacing the first component of the first signal based on the second linear polarization of the first component.

Still other aspects, features, and advantages are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the disclosure. Other implementations are also capable of other and different features and advantages, and their several details can be modified in various obvious respects, all without departing from the spirit and scope of the disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements and in which:

FIG. 1A is a schematic graph that illustrates the example transmitted signal of a series of binary digits along with returned optical signals for measurement of range, according to an implementation;

FIG. 1B is a schematic graph that illustrates an example spectrum of the reference signal and an example spectrum of a Doppler shifted return signal, according to an implementation;

FIG. 1C is a schematic graph that illustrates an example cross-spectrum of phase components of a Doppler shifted return signal, according to an implementation;

FIG. 1D is a set of graphs that illustrates an example optical chirp measurement of range, according to an implementation;

FIG. 1E is a graph using a symmetric LO signal, and shows the return signal in this frequency time plot as a dashed line when there is no Doppler shift, according to an implementation;

FIG. 1F is a graph similar to FIG. 1E, using a symmetric LO signal, and shows the return signal in this frequency time plot as a dashed line when there is a non zero Doppler shift, according to an implementation;

FIG. 2A is a block diagram that illustrates example components of a high resolution (hi res) LIDAR system, according to an implementation;

FIG. 2B is a block diagram that illustrates a saw tooth scan pattern for a hi-res Doppler system, used in some implementations;

FIG. 2C is an image that illustrates an example speed point cloud produced by a hi-res Doppler LIDAR system, according to an implementation;

FIG. 2D is a block diagram that illustrates example components of a high resolution (hi res) LIDAR system, according to an implementation;

FIG. 2E is a block diagram that illustrates a side view of example components of a bistatic LIDAR system, according to an implementation;

FIG. 2F is a block diagram that illustrates a top view of the example components of FIG. 2E, according to an implementation;

FIG. 2G is a block diagram that illustrates a top view of example components of a monostatic LIDAR system, according to an implementation;

FIG. 2H is a block diagram that illustrates a top view of example components of a monostatic LIDAR system, according to an implementation;

FIG. 2I is a block diagram that illustrates a top view of example components of a bistatic LIDAR system, according to an implementation;

FIG. 2J is a block diagram that illustrates a top view of example components of a bistatic LIDAR system, according to an implementation;

FIG. 2K is a block diagram that illustrates a top view of example components of a bistatic LIDAR system, according to an implementation;

FIG. 2L is a block diagram that illustrates a front view of the bistatic transceiver of the LIDAR system of FIG. 2I, according to an implementation;

FIG. 2M is a block diagram that illustrates a front view of the bistatic transceiver of the LIDAR system of FIG. 2I, according to an implementation;

FIG. 2N is a block diagram that illustrates a top view of example components of a bistatic LIDAR system, according to an implementation;

FIG. 2O is a block diagram that illustrates a top view of example components of a bistatic LIDAR system, according to an implementation;

FIG. 3A is a block diagram that illustrates an example system that includes at least one hi-res LIDAR system mounted on a vehicle, according to an implementation;

FIG. 3B is a block diagram that illustrates an example system that includes at least one hi-res LIDAR system mounted on a vehicle, according to an implementation;

FIG. 3C is a block diagram that illustrates an example of transmitted beams at multiple angles from the LIDAR system of FIG. 3B, according to an implementation;

FIG. 3D is a block diagram that illustrates an example system that includes at least one hi-res LIDAR system mounted on a vehicle, according to an implementation;

FIG. 4A is a graph that illustrates an example signal-to-noise ratio (SNR) versus target range for the transmitted signal in the system of FIG. 2D without scanning, according to an implementation;

FIG. 4B is a graph that illustrates an example of a curve indicating a 1/r-squared loss that drives the shape of the SNR curve of FIG. 4A in the far field, according to an implementation;

FIG. 4C is a graph that illustrates an example of collimated beam diameter versus range for the transmitted signal in the system of FIG. 2D without scanning, according to an implementation;

FIG. 4D is a graph that illustrates an example of SNR associated with collection efficiency versus range for the transmitted signal in the system of FIG. 2D without scanning, according to an implementation;

FIG. 4E is an image that illustrates an example of beam walkoff for various target ranges and scan speeds in the system of FIG. 2E, according to an implementation;

FIG. 4F is a graph that illustrates an example of coupling efficiency versus target range for various scan rates in the system of FIG. 2E, according to an implementation;

FIG. 4G is a graph that illustrates an example of SNR versus target range for various scan rates in the system of FIG. 2E, according to an implementation;

FIG. 4H is a graph that illustrates an example of a conventional scan trajectory of the beam in the system of FIG. 2E mounted on a moving vehicle, according to an implementation;

FIG. 4I is a graph that illustrates an example of SNR versus target range for various integration times in the system of FIG. 2E, according to an implementation;

FIG. 4J is a graph that illustrates an example of a measurement rate versus target range in the system of FIG. 2E, according to an implementation;

FIG. 4K is a graph that illustrates an example of SNR versus target range for various separation values in the system of FIG. 2E, according to an implementation;

FIG. 4L is a graph that illustrates an example of separation versus target range for various SNR values in the system of FIG. 2E, according to an implementation;

FIG. 4M is a graph that illustrates an example of SNR versus target range for various separation values in the system of FIG. 2E, according to an implementation;

FIG. 4N is a graph that illustrates an example of separation versus target range for various SNR values in the system of FIG. 2E, according to an implementation;

FIG. 4O is a graph that illustrates an example of separation versus scan speed for various target range values with minimum threshold SNR in the system of FIG. 2E, according to an implementation;

FIG. 5 is a graph that illustrates an example of the vertical angle over time over multiple angle ranges in the system of FIG. 2E, according to an implementation;

FIG. 6A is a flow chart that illustrates an example method for optimizing a scan pattern of a LIDAR system, according to an implementation;

FIG. 6B is a flow chart that illustrates an example method for spatial displacement of transmit and receive modes in a bistatic LIDAR system, according to an implementation;

FIG. 7 is a block diagram that illustrates a computer system upon which an embodiment of the disclosure may be implemented; and

FIG. 8 illustrates a chip set upon which an embodiment of the disclosure may be implemented.

DETAILED DESCRIPTION

Implementations of the present disclosure are generally related to systems and methods for providing optical detection of range using LIDAR, including but not limited to systems and methods for providing spatial displacement of transmit and receive modes in coherent LIDAR.

To achieve acceptable range accuracy and detection sensitivity, direct long range LIDAR systems use short pulse lasers with low pulse repetition rate and extremely high pulse peak power. The high pulse power can lead to rapid degradation of optical components. Chirped and phase-encoded LIDAR systems use long optical pulses with relatively low peak optical power. In this configuration, the range accuracy increases with the chirp bandwidth or length and bandwidth of the phase codes rather than the pulse duration, and therefore excellent range accuracy can still be obtained.

Useful optical bandwidths have been achieved using wideband radio frequency (RF) electrical signals to modulate an optical carrier. Recent advances in LIDAR include using the same modulated optical carrier as a reference signal that is combined with the returned signal at an optical detector to produce in the resulting electrical signal a relatively low beat frequency in the RF band that is proportional to the difference in frequencies or phases between the references and returned optical signals. This kind of beat frequency detection of frequency differences at a detector is called heterodyne detection. It has several advantages, such as the advantage of using RF components of ready and inexpensive availability.

Recent work by current inventors, shows a novel arrangement of optical components and coherent processing to detect Doppler shifts in returned signals that provide not only improved range but also relative signed speed on a vector between the LIDAR system and each external object. These systems are called hi-res range-Doppler LIDAR herein. These improvements can provide range, with or without target speed, in a pencil thin laser beam of proper frequency or phase content. When such beams are swept over a scene, information about the location and speed of surrounding objects can be obtained. This information is expected to be of value in control systems for autonomous vehicles, such as self driving, or driver assisted, automobiles.

Moreover, the inventors of the present disclosure noticed that conventional LIDAR systems, including those employing monostatic transceivers, involve alignment of a single mode (SM) fiber at a focal plane of a collimating optic. While this arrangement has the advantage of self-alignment as the transmit and receive light follow the same optical path, the inventors of the present disclosure also recognized that separation of the transmit and receive modes must be done within the SM fiber through some means of circulation. The inventors of the present disclosure also recognized that conventional LIDAR systems which employ bistatic transceivers do not provide a means for spatially separating the transmit mode and the receive mode. One drawback of this deficiency is that the necessary alignment of the transmit and receive modes with the respective transmit and receive waveguides of the bistatic transceiver in such conventional LIDAR systems is incredibly difficult.

To solve this problem, some implementations of the present disclosure provide free space optical components (free space optics) positioned external to the transmit and receive waveguides of a transceiver (e.g., a bistatic transceiver) to spatially separate the transmit and receive modes. This configuration can provide the advantage of easily align the transmit and receive modes with the respective transmit and receive waveguides in a transceiver (e.g., a bistatic transceiver. Here, the term “free space optic” refers to an optical component or device that uses light propagating in free space (e.g., air, outer space, vacuum, or the like) to wirelessly transmit data for telecommunications or computer networking.

In some implementations, an apparatus includes a transceiver (e.g., a monostatic transceiver) and a free space optic. The transceiver may be configured to transmit a first signal from a laser source along a transmission path and receive a second signal reflected by a target. The free space optic may be configured to direct the second signal along a receiving path such that the second signal is not received at the transmission path. This configuration can provide the advantages of transmitting a transmit beam only from a transmission path and receiving a return beam only at a receiving path, in a transceiver (e.g., a monostatic transceiver) so that transmit and receive modes are separated within the transceiver.

Hereinafter, methods, apparatuses, systems and computer-readable medium are described for scanning of a LIDAR system. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be apparent, however, to one skilled in the art that the present disclosure may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present disclosure.

Some implementations of the disclosure are described below in the context of a hi-res LIDAR system. One implementation of the disclosure is described in the context of hi-res bistatic LIDAR system. Other implementations of the disclosure are described in the context of single front mounted hi-res Doppler LIDAR system on a personal automobile; but, implementations are not limited to this context. In other implementations, one or multiple systems of the same type or other high resolution LIDAR, with or without Doppler components, with overlapping or non-overlapping fields of view or one or more such systems mounted on smaller or larger land, sea, air or space vehicles, piloted or autonomous, are employed. Static-land based LIDAR-scanning is also applicable to the implementations of the disclosure described below.

For purposes of this description, “bistatic LIDAR system” means a LIDAR system that employs a bistatic transceiver to transmit a beam from a transmission waveguide at a target and to receive one or more return beams reflected from the target at one or more receiving waveguides. Additionally, for purposes of this description, “bistatic transceiver” means a waveguide array that includes at least one transmission waveguide and at least one receiving waveguide. In some implementations, the bistatic transceiver is one or more pairs of waveguides, where each pair of waveguides is one transmission waveguide and one receiving waveguide. In other implementations, the bistatic transceiver is one or more groups of waveguides where each group includes one transmission waveguide, a first receiving waveguide spaced apart from a first side of the waveguide by a first separation and a second receiving waveguide spaced apart from a second side of the transmission waveguide by a second separation. In still other implementations, the waveguides of the bistatic transceiver are arranged in a one-dimensional array or two-dimensional array of waveguides.

1. Phase-Encoded Detection Overview

Using an optical phase-encoded signal for measurement of range, the transmitted signal is in phase with a carrier (phase=0) for part of the transmitted signal and then changes by one or more phases changes represented by the symbol Δϕ (so phase=Δϕ) for short time intervals, switching back and forth between the two or more phase values repeatedly over the transmitted signal. The shortest interval of constant phase is a parameter of the encoding called pulse duration τ and is typically the duration of several periods of the lowest frequency in the band. The reciprocal, 1/τ, is baud rate, where each baud indicates a symbol. The number N of such constant phase pulses during the time of the transmitted signal is the number N of symbols and represents the length of the encoding. In binary encoding, there are two phase values and the phase of the shortest interval can be considered a 0 for one value and a 1 for the other, thus the symbol is one bit, and the baud rate is also called the bit rate. In multiphase encoding, there are multiple phase values. For example, 4 phase values such as Δϕ*{0, 1, 2 and 3}, which, for Δϕ=π/2 (90 degrees), equals {0, π/2, π and 3π/2}, respectively; and, thus 4 phase values can represent 0, 1, 2, 3, respectively. In this example, each symbol is two bits and the bit rate is twice the baud rate.

Phase-shift keying (PSK) refers to a digital modulation scheme that conveys data by changing (modulating) the phase of a reference signal (the carrier wave). The modulation is impressed by varying the sine and cosine inputs at a precise time. At radio frequencies (RF), PSK is widely used for wireless local area networks (LANs), RF identification (RFID) and Bluetooth communication. Alternatively, instead of operating with respect to a constant reference wave, the transmission can operate with respect to itself. Changes in phase of a single transmitted waveform can be considered the symbol. In this system, the demodulator determines the changes in the phase of the received signal rather than the phase (relative to a reference wave) itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement in communications applications than ordinary PSK, since there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (thus, it is a non-coherent scheme).

For optical ranging applications, since the transmitter and receiver are in the same device, coherent PSK can be used. The carrier frequency is an optical frequency fc and a RF f₀ is modulated onto the optical carrier. The number N and duration τ of symbols are selected to achieve the desired range accuracy and resolution. The pattern of symbols is selected to be distinguishable from other sources of coded signals and noise. Thus a strong correlation between the transmitted and returned signal is a strong indication of a reflected or backscattered signal. The transmitted signal is made up of one or more blocks of symbols, where each block is sufficiently long to provide strong correlation with a reflected or backscattered return even in the presence of noise. In the following discussion, it is assumed that the transmitted signal is made up of M blocks of N symbols per block, where M and N are non-negative integers.

FIG. 1A is a schematic graph 120 that illustrates the example transmitted signal as a series of binary digits along with returned optical signals for measurement of range, according to an implementation. The horizontal axis 122 indicates time in arbitrary units after a start time at zero. The vertical axis 124 a indicates amplitude of an optical transmitted signal at frequency fc+f₀ in arbitrary units relative to zero. The vertical axis 124 b indicates amplitude of an optical returned signal at frequency fc+f₀ in arbitrary units relative to zero, and is offset from axis 124 a to separate traces. Trace 125 represents a transmitted signal of M*N binary symbols, with phase changes as shown in FIG. 1A to produce a code starting with 00011010 and continuing as indicated by ellipsis. Trace 126 represents an idealized (noiseless) return signal that is scattered from an object that is not moving (and thus the return is not Doppler shifted). The amplitude is reduced, but the code 00011010 is recognizable. Trace 127 represents an idealized (noiseless) return signal that is scattered from an object that is moving and is therefore Doppler shifted. The return is not at the proper optical frequency fc+f₀ and is not well detected in the expected frequency band, so the amplitude is diminished.

The observed frequency f′ of the return differs from the correct frequency f=fc+f₀ of the return by the Doppler effect given by Equation 1.

$\begin{matrix} {f^{\prime} = {\frac{\left( {c + v_{o}} \right)}{\left( {c + v_{s}} \right)}f}} & (1) \end{matrix}$ Where c is the speed of light in the medium, v_(o) is the velocity of the observer and vs is the velocity of the source along the vector connecting source to receiver. Note that the two frequencies are the same if the observer and source are moving at the same speed in the same direction on the vector between the two. The difference between the two frequencies, Δf=f′−f, is the Doppler shift, Δf_(D), which causes problems for the range measurement, and is given by Equation 2.

$\begin{matrix} {{\Delta f_{D}} = {\left\lbrack {\frac{\left( {c + v_{o}} \right)}{\left( {c + v_{s}} \right)} - 1} \right\rbrack f}} & (2) \end{matrix}$ Note that the magnitude of the error increases with the frequency f of the signal. Note also that for a stationary LIDAR system (v_(o)=0), for an object moving at 10 meters a second (v_(s)=10), and visible light of frequency about 500 THz, then the size of the error is on the order of 16 megahertz (MHz, 1 MHz=10⁶ hertz, Hz, 1 Hz=1 cycle per second). In various implementations described below, the Doppler shift error is detected and used to process the data for the calculation of range.

In phase coded ranging, the arrival of the phase coded reflection is detected in the return by cross correlating the transmitted signal or other reference signal with the returned signal, implemented practically by cross correlating the code for a RF signal with an electrical signal from an optical detector using heterodyne detection and thus down-mixing back to the RF band. Cross correlation for any one lag is computed by convolving the two traces, i.e., multiplying corresponding values in the two traces and summing over all points in the trace, and then repeating for each time lag. Alternatively, the cross correlation can be accomplished by a multiplication of the Fourier transforms of each of the two traces followed by an inverse Fourier transform. Efficient hardware and software implementations for a Fast Fourier transform (FFT) are widely available for both forward and inverse Fourier transforms.

Note that the cross correlation computation is typically done with analog or digital electrical signals after the amplitude and phase of the return is detected at an optical detector. To move the signal at the optical detector to a RF frequency range that can be digitized easily, the optical return signal is optically mixed with the reference signal before impinging on the detector. A copy of the phase-encoded transmitted optical signal can be used as the reference signal, but it is also possible, and often preferable, to use the continuous wave carrier frequency optical signal output by the laser as the reference signal and capture both the amplitude and phase of the electrical signal output by the detector.

For an idealized (noiseless) return signal that is reflected from an object that is not moving (and thus the return is not Doppler shifted), a peak occurs at a time Δt after the start of the transmitted signal. This indicates that the returned signal includes a version of the transmitted phase code beginning at the time Δt. The range R to the reflecting (or backscattering) object is computed from the two way travel time delay based on the speed of light c in the medium, as given by Equation 3. R=c*Δt/2  (3)

For an idealized (noiseless) return signal that is scattered from an object that is moving (and thus the return is Doppler shifted), the return signal does not include the phase encoding in the proper frequency bin, the correlation stays low for all time lags, and a peak is not as readily detected, and is often undetectable in the presence of noise. Thus Δt is not as readily determined and range R is not as readily produced.

According to various implementations of the inventor's previous work, the Doppler shift is determined in the electrical processing of the returned signal; and the Doppler shift is used to correct the cross correlation calculation. Thus a peak is more readily found and range can be more readily determined. FIG. 1B is a schematic graph 140 that illustrates an example spectrum of the transmitted signal and an example spectrum of a Doppler shifted complex return signal, according to an implementation. The horizontal axis 142 indicates RF frequency offset from an optical carrier fc in arbitrary units. The vertical axis 144 a indicates amplitude of a particular narrow frequency bin, also called spectral density, in arbitrary units relative to zero. The vertical axis 144 b indicates spectral density in arbitrary units relative to zero, and is offset from axis 144 a to separate traces. Trace 145 represents a transmitted signal; and, a peak occurs at the proper RF f₀. Trace 146 represents an idealized (noiseless) complex return signal that is backscattered from an object that is moving toward the LIDAR system and is therefore Doppler shifted to a higher frequency (called blue shifted). The return does not have a peak at the proper RF f₀; but, instead, is blue shifted by Δf_(D) to a shifted frequency f_(S). In practice, a complex return representing both in-phase and quadrature (I/Q) components of the return is used to determine the peak at +Δf_(D), thus the direction of the Doppler shift, and the direction of motion of the target on the vector between the sensor and the object, is apparent from a single return.

In some Doppler compensation implementations, rather than finding Δf_(D) by taking the spectrum of both transmitted and returned signals and searching for peaks in each, then subtracting the frequencies of corresponding peaks, as illustrated in FIG. 1B, it is more efficient to take the cross spectrum of the in-phase and quadrature component of the down-mixed returned signal in the RF band. FIG. 1C is a schematic graph 150 that illustrates an example cross-spectrum, according to an implementation. The horizontal axis 152 indicates frequency shift in arbitrary units relative to the reference spectrum; and, the vertical axis 154 indicates amplitude of the cross spectrum in arbitrary units relative to zero. Trace 155 represents a cross spectrum with an idealized (noiseless) return signal generated by one object moving toward the LIDAR system (blue shift of Δf_(D1)=Δf_(D) in FIG. 1B) and a second object moving away from the LIDAR system (red shift of Δf_(D2)). A peak occurs when one of the components is blue shifted Δf_(D1); and, another peak occurs when one of the components is red shifted Δf_(D2). Thus the Doppler shifts are determined. These shifts can be used to determine a signed velocity of approach of objects in the vicinity of the LIDAR, as can be critical for collision avoidance applications. However, if I/Q processing is not done, peaks appear at both +/−Δf_(D1) and both +/−Δf_(D2), so there is ambiguity on the sign of the Doppler shift and thus the direction of movement.

As described in more detail in inventor's previous work the Doppler shift(s) detected in the cross spectrum are used to correct the cross correlation so that the peak 135 is apparent in the Doppler compensated Doppler shifted return at lag Δt, and range R can be determined. In some implementations, simultaneous I/Q processing is performed as described in more detail in patent application publication entitled “Method and system for Doppler detection and Doppler correction of optical phase-encoded range detection” by S. Crouch et al., the entire contents of which are herby incorporated by reference as if fully set forth herein. In other implementations, serial UQ processing is used to determine the sign of the Doppler return as described in more detail in patent application publication entitled “Method and System for Time Separated Quadrature Detection of Doppler Effects in Optical Range Measurements” by S. Crouch et al., the entire contents of which are herby incorporated by reference as if fully set forth herein. In other implementations, other means are used to determine the Doppler correction; and, in various implementations, any method known in the art to perform Doppler correction is used. In some implementations, errors due to Doppler shifting are tolerated or ignored; and, no Doppler correction is applied to the range measurements.

2. Chirped Detection Overview

FIG. 1D is a set of graphs that illustrates an example optical chirp measurement of range, according to an implementation. The horizontal axis 102 is the same for all four graphs and indicates time in arbitrary units, on the order of milliseconds (ms, 1 ms=10⁻³ seconds). Graph 100 indicates the power of a beam of light used as a transmitted optical signal. The vertical axis 104 in graph 100 indicates power of the transmitted signal in arbitrary units. Trace 106 indicates that the power is on for a limited pulse duration, r starting at time 0. Graph 110 indicates the frequency of the transmitted signal. The vertical axis 114 indicates the frequency transmitted in arbitrary units. The trace 116 indicates that the frequency of the pulse increases from f₁ to f₂ over the duration τ of the pulse, and thus has a bandwidth B=f₂−f₁. The frequency rate of change is (f₂−f₁)/τ.

The returned signal is depicted in graph 160 which has a horizontal axis 102 that indicates time and a vertical axis 114 that indicates frequency as in graph 110. The chirp 116 of graph 110 is also plotted as a dotted line on graph 160. A first returned signal is given by trace 166 a, which is just the transmitted reference signal diminished in intensity (not shown) and delayed by Δt. When the returned signal is received from an external object after covering a distance of 2R, where R is the range to the target, the returned signal start at the delayed time Δt is given by 2R/c, where c is the speed of light in the medium (approximately 3×10⁸ meters per second, m/s), related according to Equation 3, described above. Over this time, the frequency has changed by an amount that depends on the range, calledfR, and given by the frequency rate of change multiplied by the delay time. This is given by Equation 4a. f _(R)=(f ₂ −f ₁)/τ*2R/c=2BR/cτ  (4a) The value of f_(R) is measured by the frequency difference between the transmitted signal 116 and returned signal 166 a in a time domain mixing operation referred to as de-chirping. So the range R is given by Equation 4b. R=f _(R) cτ/2B  (4b) Of course, if the returned signal arrives after the pulse is completely transmitted, that is, if 2R/c is greater than τ, then Equations 4a and 4b are not valid. In this case, the reference signal is delayed a known or fixed amount to ensure the returned signal overlaps the reference signal. The fixed or known delay time of the reference signal is multiplied by the speed of light, c, to give an additional range that is added to range computed from Equation 4b. While the absolute range may be off due to uncertainty of the speed of light in the medium, this is a near-constant error and the relative ranges based on the frequency difference are still very precise.

In some circumstances, a spot illuminated (pencil beam cross section) by the transmitted light beam encounters two or more different scatterers at different ranges, such as a front and a back of a semitransparent object, or the closer and farther portions of an object at varying distances from the LIDAR, or two separate objects within the illuminated spot. In such circumstances, a second diminished intensity and differently delayed signal will also be received, indicated on graph 160 by trace 166 b. This will have a different measured value of f_(R) that gives a different range using Equation 4b. In some circumstances, multiple additional returned signals are received.

Graph 170 depicts the difference frequency f_(R) between a first returned signal 166 a and the reference chirp 116. The horizontal axis 102 indicates time as in all the other aligned graphs in FIG. 1D, and the vertical axis 164 indicates frequency difference on a much expanded scale. Trace 176 depicts the constant frequency f_(R) measured in response to the transmitted chirp, which indicates a particular range as given by Equation 4b. The second returned signal 166 b, if present, would give rise to a different, larger value of f_(R) (not shown) during de-chirping; and, as a consequence yield a larger range using Equation 4b.

A common method for de-chirping is to direct both the reference optical signal and the returned optical signal to the same optical detector. The electrical output of the detector is dominated by a beat frequency that is equal to, or otherwise depends on, the difference in the frequencies of the two signals converging on the detector. A Fourier transform of this electrical output signal will yield a peak at the beat frequency. This beat frequency is in the radio frequency (RF) range of Megahertz (MHz, 1 MHz=10⁶ Hertz=10⁶ cycles per second) rather than in the optical frequency range of Terahertz (THz, 1 THz=10¹² Hertz). Such signals are readily processed by common and inexpensive RF components, such as a Fast Fourier Transform (FFT) algorithm running on a microprocessor or a specially built FFT or other digital signal processing (DSP) integrated circuit. In other implementations, the return signal is mixed with a continuous wave (CW) tone acting as the local oscillator (versus a chirp as the local oscillator). This leads to the detected signal which itself is a chirp (or whatever waveform was transmitted). In this case the detected signal would undergo matched filtering in the digital domain as described in Kachelmyer 1990, the entire contents of which are hereby incorporated by reference as if fully set forth herein, except for terminology inconsistent with that used herein. The disadvantage is that the digitizer bandwidth requirement is generally higher. The positive aspects of coherent detection are otherwise retained.

In some implementations, the LIDAR system is changed to produce simultaneous up and down chirps. This approach eliminates variability introduced by object speed differences, or LIDAR position changes relative to the object which actually does change the range, or transient scatterers in the beam, among others, or some combination. The approach then guarantees that the Doppler shifts and ranges measured on the up and down chirps are indeed identical and can be most usefully combined. The Doppler scheme guarantees parallel capture of asymmetrically shifted return pairs in frequency space for a high probability of correct compensation.

FIG. 1E is a graph using a symmetric LO signal, and shows the return signal in this frequency time plot as a dashed line when there is no Doppler shift, according to an implementation. The horizontal axis indicates time in example units of 10⁻⁵ seconds (tens of microseconds). The vertical axis indicates frequency of the optical transmitted signal relative to the carrier frequency f_(c) or reference signal in example units of GigaHertz (10⁹ Hertz). During a pulse duration, a light beam comprising two optical frequencies at any time is generated. One frequency increases from f₁ to f₂ (e.g., 1 to 2 GHz above the optical carrier) while the other frequency simultaneous decreases from f₄ to f₃ (e.g., 1 to 2 GHz below the optical carrier) The two frequency bands e.g., band 1 from f₁ to f₂, and band 2 from f₃ to f₄) do not overlap so that both transmitted and return signals can be optically separated by a high pass or a low pass filter, or some combination, with pass bands starting at pass frequency f_(p). For example f₁<f₂<f_(p)<f₃<f₄. Though, in the illustrated implementation, the higher frequencies provide the up chirp and the lower frequencies provide the down chirp, in other implementations, the higher frequencies produce the down chirp and the lower frequencies produce the up chirp.

In some implementations, two different laser sources are used to produce the two different optical frequencies in each beam at each time. However, in some implementations, a single optical carrier is modulated by a single RF chirp to produce symmetrical sidebands that serve as the simultaneous up and down chirps. In some of these implementations, a double sideband Mach-Zehnder intensity modulator is used that, in general, does not leave much energy in the carrier frequency; instead, almost all of the energy goes into the sidebands.

As a result of sideband symmetry, the bandwidth of the two optical chirps will be the same if the same order sideband is used. In other implementations, other sidebands are used, e.g., two second order sideband are used, or a first order sideband and a non-overlapping second sideband is used, or some other combination.

As described in U.S. patent application publication by Crouch et al., entitled “Method and System for Doppler Detection and Doppler Correction of Optical Chirped Range Detection,” when selecting the transmit (TX) and local oscillator (LO) chirp waveforms, it is advantageous to ensure that the frequency shifted bands of the system take maximum advantage of available digitizer bandwidth. In general this is accomplished by shifting either the up chirp or the down chirp to have a range frequency beat close to zero.

FIG. 1F is a graph similar to FIG. 1E, using a symmetric LO signal, and shows the return signal in this frequency time plot as a dashed line when there is a non zero Doppler shift. In the case of a chirped waveform, the time separated I/Q processing (aka time domain multiplexing) can be used to overcome hardware requirements of other approaches as described above. In that case, an AOM is used to break the range-Doppler ambiguity for real valued signals. In some implementations, a scoring system is used to pair the up and down chirp returns as described in more detail in patent application publication entitled “Method and system for Doppler detection and Doppler correction of optical chirped range detection” by S. Crouch et al., the entire contents of which are hereby incorporated by reference as if fully set forth herein. In other implementations, I/Q processing is used to determine the sign of the Doppler chirp as described in more detail in patent application publication entitled “Method and System for Time Separated Quadrature Detection of Doppler Effects in Optical Range Measurements” by S. Crouch et al., the entire contents of which are hereby incorporated by reference as if fully set forth herein.

3. Optical Detection Hardware Overview

In order to depict how to use hi-res range-Doppler detection systems, some generic hardware approaches are described. FIG. 2A is a block diagram that illustrates example components of a high resolution range LIDAR system 200, according to an implementation. Optical signals are indicated by arrows. Electronic wired or wireless connections are indicated by segmented lines without arrowheads. A laser source 212 emits a carrier wave 201 that is phase or frequency modulated in modulator 282 a, before or after splitter 216, to produce a phase coded or chirped optical signal 203 that has a duration D. A splitter 216 splits the modulated (or, as shown, the unmodulated) optical signal for use in a reference path 220. A target beam 205, also called transmitted signal herein, with most of the energy of the beam 201 is produced. A modulated or unmodulated reference beam 207 a with a much smaller amount of energy that is nonetheless enough to produce good mixing with the returned light 291 scattered from an object (not shown) is also produced. In the illustrated implementation, the reference beam 207 a is separately modulated in modulator 282 b. The reference beam 207 a passes through reference path 220 and is directed to one or more detectors as reference beam 207 b. In some implementations, the reference path 220 introduces a known delay sufficient for reference beam 207 b to arrive at the detector array 230 with the scattered light from an object outside the LIDAR within a spread of ranges of interest. In some implementations, the reference beam 207 b is called the local oscillator (LO) signal referring to older approaches that produced the reference beam 207 b locally from a separate oscillator. In various implementations, from less to more flexible approaches, the reference is caused to arrive with the scattered or reflected field by: 1) putting a mirror in the scene to reflect a portion of the transmit beam back at the detector array so that path lengths are well matched; 2) using a fiber delay to closely match the path length and broadcast the reference beam with optics near the detector array, as suggested in FIG. 2A, with or without a path length adjustment to compensate for the phase or frequency difference observed or expected for a particular range; or, 3) using a frequency shifting device (acousto-optic modulator) or time delay of a local oscillator waveform modulation (e.g., in modulator 282 b) to produce a separate modulation to compensate for path length mismatch; or some combination. In some implementations, the object is close enough and the transmitted duration long enough that the returns sufficiently overlap the reference signal without a delay.

The transmitted signal is then transmitted to illuminate an area of interest, often through some scanning optics 218. The detector array is a single paired or unpaired detector or a 1 dimensional (1D) or 2 dimensional (2D) array of paired or unpaired detectors arranged in a plane roughly perpendicular to returned beams 291 from the object. The reference beam 207 b and returned beam 291 are combined in zero or more optical mixers 284 to produce an optical signal of characteristics to be properly detected. The frequency, phase or amplitude of the interference pattern, or some combination, is recorded by acquisition system 240 for each detector at multiple times during the signal duration D. The number of temporal samples processed per signal duration or integration time affects the down-range extent. The number or integration time is often a practical consideration chosen based on number of symbols per signal, signal repetition rate and available camera frame rate. The frame rate is the sampling bandwidth, often called “digitizer frequency.” The only fundamental limitations of range extent are the coherence length of the laser and the length of the chirp or unique phase code before it repeats (for unambiguous ranging). This is enabled because any digital record of the returned heterodyne signal or bits could be compared or cross correlated with any portion of transmitted bits from the prior transmission history.

The acquired data is made available to a processing system 250, such as a computer system described below with reference to FIG. 7, or a chip set described below with reference to FIG. 8. A scanner control module 270 provides scanning signals to drive the scanning optics 218, according to one or more of the implementations described below. In one implementation, the scanner control module 270 includes instructions to perform one or more steps of the method 600 related to the flowchart of FIG. 6A and/or one or more steps of the method 650 related to the flowchart of FIG. 6B. A signed Doppler compensation module (not shown) in processing system 250 determines the sign and size of the Doppler shift and the corrected range based thereon along with any other corrections. The processing system 250 also includes a modulation signal module (not shown) to send one or more electrical signals that drive modulators 282 a, 282 b. In some implementations, the processing system also includes a vehicle control module 272 to control a vehicle on which the system 200 is installed.

Any known apparatus or system may be used to implement the laser source 212, modulators 282 a, 282 b, beam splitter 216, reference path 220, optical mixers 284, detector array 230, scanning optics 218, or acquisition system 240. Optical coupling to flood or focus on a target or focus past the pupil plane are not depicted. As used herein, an optical coupler is any component that affects the propagation of light within spatial coordinates to direct light from one component to another component, such as a vacuum, air, glass, crystal, mirror, lens, optical circulator, beam splitter, phase plate, polarizer, optical fiber, optical mixer, among others, alone or in some combination.

FIG. 2A also illustrates example components for a simultaneous up and down chirp LIDAR system according to one implementation. In this implementation, the modulator 282 a is a frequency shifter added to the optical path of the transmitted beam 205. In other implementations, the frequency shifter is added instead to the optical path of the returned beam 291 or to the reference path 220. In general, the frequency shifting element is added as modulator 282 b on the local oscillator (LO, also called the reference path) side or on the transmit side (before the optical amplifier) as the device used as the modulator (e.g., an acousto-optic modulator, AOM) has some loss associated and it is disadvantageous to put lossy components on the receive side or after the optical amplifier. The purpose of the optical shifter is to shift the frequency of the transmitted signal (or return signal) relative to the frequency of the reference signal by a known amount Δfs, so that the beat frequencies of the up and down chirps occur in different frequency bands, which can be picked up, e.g., by the FFT component in processing system 250, in the analysis of the electrical signal output by the optical detector 230. For example, if the blue shift causing range effects is f_(B), then the beat frequency of the up chirp will be increased by the offset and occur at f_(B)+Δfs and the beat frequency of the down chirp will be decreased by the offset to f_(B)-Δfs. Thus, the up chirps will be in a higher frequency band than the down chirps, thereby separating them. If Δfs is greater than any expected Doppler effect, there will be no ambiguity in the ranges associated with up chirps and down chirps. The measured beats can then be corrected with the correctly signed value of the known Δfs to get the proper up-chirp and down-chirp ranges. In some implementations, the RF signal coming out of the balanced detector is digitized directly with the bands being separated via FFT. In some implementations, the RF signal coming out of the balanced detector is pre-processed with analog RF electronics to separate a low-band (corresponding to one of the up chirp or down chip) which can be directly digitized and a high-band (corresponding to the opposite chirp) which can be electronically down-mixed to baseband and then digitized. Both implementations offer pathways that match the bands of the detected signals to available digitizer resources. In some implementations, the modulator 282 a is excluded (e.g. direct ranging).

FIG. 2B is a block diagram that illustrates a simple saw tooth scan pattern for a hi-res Doppler system, used in some implementations. The scan sweeps through a range of azimuth angles (horizontally) and inclination angles (vertically above and below a level direction at zero inclination). In various implementations described below, other scan patterns are used. Any scan pattern known in the art may be used in various implementations. For example, in some implementations, adaptive scanning is performed using methods described in PCT patent applications by Crouch entitled “Method and system for adaptive scanning with optical ranging systems,” or entitled “Method and system for automatic real-time adaptive scanning with optical ranging systems,” the entire contents of each of which are hereby incorporated by reference as if fully set forth herein. FIG. 2C is an image that illustrates an example speed point cloud produced by a hi-res Doppler LIDAR system, according to an implementation.

FIG. 2D is a block diagram that illustrates example components of a high resolution (hi res) LIDAR system 200′, according to an implementation. In an implementation, the system 200′ is similar to the system 200 with the exception of the features discussed herein. In an implementation, the system 200′ is a coherent LIDAR system that is constructed with monostatic transceivers. The system 200′ includes the source 212 that transmits the carrier wave 201 along a single-mode optical waveguide over a transmission path 222, through a free space optic 226 and out a tip 217 of the single-mode optical waveguide that is positioned in a focal plane of a collimating optic 229. In some implementations, the free space optic 226 may be an optical circulator but is not limited thereto. In an implementation, the tip 217 is positioned within a threshold distance (e.g. about 100 μm) of the focal plane of the collimating optic 229 or within a range from about 0.1% to about 0.5% of the focal length of the collimating optic 229. In another implementation, the collimating optic 229 includes one or more of doublets, aspheres or multi-element designs. In an implementation, the carrier wave 201 exiting the optical waveguide tip 217 is shaped by the optic 229 into a collimated target beam 205′ which is scanned over a range of angles 227 by scanning optics 218. In some implementations, the carrier wave 201 is phase or frequency modulated in a modulator 282 a upstream of the collimation optic 229. In other implementations, modulator 282 is excluded. In an implementation, return beams 291 from an object are directed by the scanning optics 218 and focused by the collimation optics 229 onto the tip 217 so that the return beam 291 is received in the single-mode optical waveguide tip 217. In an implementation, the return beam 291 is then redirected by the free space optic 226 into a single mode optical waveguide along the receiving path 224 and to optical mixers 284 where the return beam 291 is combined with the reference beam 207 b that is directed through a single-mode optical waveguide along a local oscillator path 220. In one implementation, the system 200′ operates under the principal that maximum spatial mode overlap of the returned beam 291 with the reference signal 207 b will maximize heterodyne mixing (optical interference) efficiency between the returned signal 291 and the local oscillator 207 b. This monostatic arrangement is advantageous as it can help to avoid challenging alignment procedures associated with bi-static LIDAR systems.

FIG. 2E is a block diagram that illustrates a side view of example components of a bistatic LIDAR system 200″, according to an implementation. FIG. 2F is a block diagram that illustrates a top view of the example components of the bistatic LIDAR system 200″ of FIG. 2E, according to an implementation. In an implementation, the bistatic LIDAR system 200″ is similar to the system 200′ of FIG. 2D with the exception of the features discussed herein.

In an implementation, the system 200″ includes a bistatic transceiver 215 that includes a transmission waveguide 223 and one or more receiving waveguides 225 a, 225 b. The first receiving waveguide 225 a is spaced apart from the transmission waveguide 223 by a separation 221 a. The second receiving waveguide 225 b is spaced apart from the transmission waveguide 223 by a separation 221 b that is greater than the spacing 221 a. Although FIG. 2F depicts two receiving waveguides 225 a, 225 b, the system is not limited to two receiving waveguides and could include one or more than two receiving waveguides. In an example implementation, the bistatic transceiver 215 is supported by on-chip waveguide technology such as planar light circuits that allow the manufacture of closely spaced waveguides to act as the bi-static transceiver aperture. In an example implementation, the bistatic transceiver 215 features planar lightwave circuit technology developed by NeoPhotonics® Corporation of San Jose, Calif. In another example implementation, the bistatic transceiver 215 is custom made with minimal modification to standard manufacturing processes of planar lightwave circuit technologies. In yet another example implementation, the bistatic transceiver 215 is manufactured by PLC Connections® of Columbus Ohio.

In an implementation, in the system 200″ the source 212 transmits the carrier wave 201 along the transmission waveguide 223 over the transmission path 222 to a tip 217 of the transmission waveguide 223. In one implementation, the system 200″ excludes the free space optic 226 which may advantageously reduce the cost and complexity of the system 200″. The carrier wave 201 exiting the tip 217 of the transmission waveguide 223 is shaped by the collimation optic 229 into the collimated target beam 205′ as in the system 200′.

In an implementation, the scanning optics 218 is a polygon scanner 244 with a plurality of mirrors or facets 245 a, 245 b and configured to rotate with an angular velocity 249 about an axis of rotation 243. In one implementation, the polygon scanner 244 is configured to rotate at a constant speed about the axis of rotation 243. In an example implementation, the polygon scanner 244 has one or more of the following characteristics: manufactured by Blackmore® Sensors with Copal turned mirrors, has an inscribed diameter of about 2 inches or in a range from about 1 inch to about 3 inches, each mirror is about 0.5 inches tall or in a range from about 0.25 inches to about 0.75 inches, has an overall height of about 2.5 inches or in a range from about 2 inches to about 3 inches, is powered by a three-phase Brushless Direct Current (BLDC) motor with encoder pole-pair switching, has a rotation speed in a range from about 1000 revolutions per minute (rpm) to about 5000 rpm, has a reduction ratio of about 5:1 and a distance from the collimator 231 of about 1.5 inches or in a range from about 1 inch to about 2 inches. In other implementations, the scanning optics 218 of the system 200″ is any optic other than the polygon scanner 244.

In an implementation, the collimated target beam 205′ is reflected off one of the polygon facets 245 into a scanned beam 205″ and is scanned through the range of angles 227 as the polygon scanner 244 rotates at the angular velocity 249. In one implementation, the bistatic transceiver 215 including the transmission waveguide 223 and receiving waveguides 225 are arranged in a first plane (e.g. plane of FIG. 2F) and the polygon scanner 244 adjusts the direction of the beam 205″ over the range of angles 227 in the same first plane (or in a plane parallel to the first plane). In another implementation, the first plane is orthogonal to the axis of rotation 243. For purposes of this description, “parallel” means within ±10 degrees and “orthogonal” means within 90±10 degrees.

In an implementation, the beam 205″ is reflected by a target positioned at a range and the return beam 291′ is reflected by one of the facets 245 to the collimation optic 229 which focuses the return beam 291′ into the tip 217 of the receiving waveguide 225 a.

In an implementation, the values of one or more parameters of the system 200″, 200′″, 200″″ are selected during a design phase of the system 200″, 200′″, 200″″ to optimize the signal to noise ratio (SNR) of the return beam 291′. In one implementation, the values of these parameters include the value of the rotation speed of the polygon scanner 244 that is selected based on a target design range over the range of angles 227 to optimize the SNR. FIG. 4G is a graph that illustrates an example of SNR versus target range for various scan rates in the system 200″ of FIG. 2E, according to an implementation. The horizontal axis 402 is target range in units of meters (m) and the vertical axis 404 is SNR in units of decibels (dB). A first curve 440 d depicts the SNR of the focused return beam 291′ on the tip 217 of the receiving waveguide 225 based on target range where the beam is not scanned. A second curve 440 b depicts the SNR of the focused return beam 291′ on the tip 217 of the receiving waveguide 225 based on target range where the beam is scanned at a slow scan rate (e.g. about 2500 degrees per second). A third curve 440 c depicts the SNR of the focused return beam 291′ on the tip 217 of the receiving waveguide 225 based on target range where the beam is scanned at an optimal scan rate (e.g. about 5500 deg/sec). An SNR threshold 442 (e.g. 10 dB) is also depicted. Thus, when designing the system 200″ a user first determines the target design range over the range of angles (e.g. 0 m-150 m) and then uses FIG. 4G to quickly determine which of the curves 440 b, 440 c, 440 d maintains an SNR above the SNR threshold 442 over that target design range. In this example implementation, the curve 440 c maintains an SNR above the SNR threshold 442 over the target design range (e.g. 0 m-150 m) and thus the user selects the optimal scan speed (e.g. about 5500 deg/sec) associated with the curve 440 c in designing the system 200″. Accordingly, the polygon scanner 244 is set to have a fixed rotation speed based on this optimal scan speed. Thus, the curves 440 advantageously provide an efficient way for a user to design a system 200″, specifically when selecting the fixed scan speed of the polygon scanner 244. In an implementation, each curve 440 is generated using the system 200″ and measuring the SNR of the return beam 291′ at each scan speed of the polygon scanner associated with each curve 440. The curves 440 are not limited to those depicted in FIG. 4G and include any SNR curves that are generated using similar means. In some implementations, curves similar to those in FIG. 4G can be generated for the system 200′″ (FIG. 2I) or system 200″″ (FIG. 2J) in a similar manner as for the system 200″ above and which provide the user with an advantageous and efficient way to design a system 200′″ or system 200″″.

In another implementation, the values of the design parameter that is selected during the design phase of the system 200″ includes the value of the separation 221 that is selected based on a scan speed of the polygon scanner 244 and a target design range over the range of angles 227 to optimize the SNR. In another implementation, the values of the design parameter that is selected during the design phase of the system 200′″ (FIG. 2I) includes the value of the separation 221 and/or length 262 of a free space optic 260. In some implementations, the free space optic 260 may be a birefringent displacer but is not limited thereto. In an implementation, the length 262 of the free space optic 260 is measured along a longitudinal axis of the system, where the longitudinal axis is defined by the direction of the transmitted beam 205′ (FIG. 2I) and/or the return beam 291′ (FIG. 2J) and/or a vector from the fiber array 215′, 215″ to the target 292. In yet another implementation, the values of the design parameter that is selected during the design phase of the system 200″″ (FIG. 2J) includes the separations 221 a, 221 b and/or the length 262 a of a first free space optic 260 a and/or the length 262 b of a second free space optic 260 b. In some implementations, each of the first and second free space optics may be a birefringent displacer but is not limited thereto. FIG. 4K is a graph that illustrates an example of SNR versus target range for various separation 221 values in the system 200″ of FIG. 2E at a low fixed scan speed (e.g. 4000 deg/sec), according to an implementation. The horizontal axis 402 is target range in units of meters (m) and the vertical axis 404 is SNR in units of decibels (dB). A first curve 464 a depicts the SNR of the focused return beam 291′ on the tip 217 of the receiving waveguide 225 based on target range for a separation 221 of 4w_(o) where w_(o) is a diameter of the transmission waveguide 223. A second curve 464 b depicts the SNR of the focused return beam 291′ on the tip 217 of the receiving waveguide 225 based on target range for a separation 221 of 0 (e.g. 0w_(o)). Each curve 464 between the first curve 464 a and second curve 464 b represents a 0.25w_(o) decrement in the separation 221 value. In one implementation, for a target design range (e.g. 0 m-250 m) the curve 464 c is selected since it has an SNR value above the SNR threshold 442 over the target design range. The separation value 221 associated with the curve 464 c is 0.25w_(o) and hence the separation 221 is set at 0.25w_(o) when designing the system 200″ with the target design range (e.g. 0 m-250 m) and the low fixed scan speed (e.g. 4000 deg/sec). In some implementations, SNR curves similar to the curves 464 of FIG. 4K can be generated based on the various separation 221 and/or length 262 values of the system 200′″ (FIG. 2I) and/or based on the various separations 221 a, 221 b and length 262 a, 262 b values of the system 200″″ (FIG. 2J). FIG. 4L is a graph that depicts a plurality of SNR curves 466 for the system 200″ with the low fixed scan speed (e.g. 4000 deg/sec). For a specific design target range (e.g. 250 m) along the horizontal axis 402, the SNR curves 466 communicate a value of the separation 221 (along the vertical axis 409) that is necessary to maintain an SNR level associated with the curve 466. In an example implementation, for the design target range of 250 m, curve 466 a indicates that an SNR of 18 dB can be maintained at multiple values of the separation 221 (e.g. about 0.25 w_(o) and about 2.75 w_(o)) and thus gives the user different options when designing the system 200″. In addition to FIG. 4K, the curves 466 provide a quick look up means for a user when designing a system 200″ based on a known design target range and fixed scan speed. In an implementation, as with the curves 464 of FIG. 4K, the curves 466 are generated using the system 200″ by measuring the SNR of the return beam 291′ at multiple separation 221 values across multiple target range values. Additionally, the curves 466 are not limited to those depicted in FIG. 4L. In some implementations, SNR curves similar to the curves 466 of FIG. 4L can be generated based on the various separation 221 and/or length 262 values of the system 200′″ and/or based on the various separations 221 a, 221 b and length 262 a, 262 b values of the system 200″″.

FIG. 4M is a graph that is similar to the graph of FIG. 4K but is for a high fixed scan speed (e.g. 12000 deg/sec). First curve 465 a is similar to first curve 464 a and is for a separation 221 of 4w_(o). Second curve 465 b is similar to second curve 464 b and is for a separation 221 of 0 (e.g. 0w_(o)). Using the same target design range (e.g. 0 m-250 m), curve 465 c is selected since it has SNR values above the SNR threshold 442 over the target design range. The separation 221 value associated with curve 465 c is 2.75w_(o) and hence the separation 221 in the system 200″ is set at 2.75w_(o) if the polygon scanner 244 is operated at the high fixed scan speed (e.g. 12000 deg/sec). Thus, when designing the system 200″ a user can first determine the target design range over the range of angles (e.g. 0 m-250 m) and then use FIG. 4M to quickly determine which of the curves 465 maintains an SNR above the SNR threshold 442 over that target design range and the fixed scan speed of the polygon scanner 244. In some implementations, SNR curves similar to the curves 465 of FIG. 4M can be generated based on the various separation 221 and/or length 262 values of the system 200′″ and/or based on the various separations 221 a, 221 b and length 262 a, 262 b values of the system 200″″. FIG. 4N is a graph that depicts a plurality of SNR curves 467 for the system 200″ with the high fixed scan speed (e.g. 12000 deg/sec). For a specific design target range (e.g. 100 m) along the horizontal axis 402, the SNR curves 467 communicate a value of the separation 221 (along the vertical axis 409) that is necessary to maintain an SNR level associated with the curve 467. In an example implementation, for the design target range of 100 m, curve 467 a indicates that an SNR of 28 dB can be maintained at multiple values of the separation 221 (e.g. about 0.75 w_(o) and about 2.25 w_(o)). In addition to FIG. 4M, the curves 467 provide a quick look up means for a user when designing a system 200″ based on a known design target range and fixed scan speed. In an implementation, as with the curves 465 of FIG. 4M, the curves 467 are generated using the system 200″ by measuring the SNR of the return beam 291′ at multiple separation 221 values across multiple target range values. Additionally, the curves 467 are not limited to those depicted in FIG. 4N. In some implementations, SNR curves similar to the curves 467 of FIG. 4N can be generated based on the various separation 221 and/or length 262 values of the system 200′″ and/or based on the various separations 221 a, 221 b and length 262 a, 262 b values of the system 200″″.

FIG. 4O is a graph that illustrates an example of separation versus scan speed for various target range values with minimum threshold SNR in the system of FIG. 2E, according to an implementation. The horizontal axis 403 is scan speed in units of kilo degrees per second. The vertical axis 409 is the separation 221 in units of w_(o). At a particular scan speed along the horizontal axis 403, the curves 469 provide the value of the separation 221 to maintain the SNR threshold 442 (e.g. 10 dB) across a design target range value associated with the curve 469. In an example implementation, for the scan speed of 12,000 deg/second, curve 469 a indicates a separation 221 value of about 2.75 w_(o) to maintain the SNR threshold 442 for a design target range of 250 m. This is consistent with the example implementation of curve 465 c of FIG. 4M. Additionally, in an example implementation, for the scan speed of 4,000 deg/second, curve 469 a indicates a separation 221 value of about 0.25 w_(o) to maintain the SNR threshold 442 for a design target range of 250 m. This is consistent with the example implementation of curve 464 c of FIG. 4K. Thus, FIG. 4O provides an additional advantageous look up graph that is useful during the design phase of the system 200″. In some implementations, curves similar to the curves 469 of FIG. 4O can be generated based on the various separation 221 and/or length 262 values of the system 200′″ and/or based on the various separations 221 a, 221 b and length 262 a, 262 b values of the system 200″″.

As depicted in FIG. 2F, the polygon scanner 244 rotates from a first orientation (e.g. solid line) to a second orientation (e.g. dotted line) during the round trip time to the target, e.g. between the time that the beam 205″ is reflected from the facet 245 a to the target and the time that the return beam 291′ is reflected by the facet 245 a to the optic 229. In one implementation, the rotation of the facet 245 a between these times accounts for the return beam 291′ being deflected by the facet 245 a at an angle 228 relative to the incident beam 205′. In an implementation, the target range (e.g. based on the round trip time) and/or the rotation speed of the polygon scanner 244 and/or a diameter of an image 418 (FIG. 4E) of the return beam 291′ on the bistatic transceiver 215 determine the angle 228 and hence the separation 221 a that is selected to position the receiving waveguide 225 a relative to the transmission waveguide 223 so the return beam 291′ is focused in the tip of the receiving waveguide 225 a. In the implementation of the system 200′″ of FIG. 2I, one or more of the above parameters are also considered in addition to the length 262 of the free space optic 260 (e.g., a birefringent displacer) and/or displacement distance 264 of the return beam 291′ so the return beam 291′ is focused in the tip of the receiving waveguide 225′. In the implementation of the system 200″″ of FIG. 2J, one or more of the above parameters are also considered in addition to the lengths 262 a, 262 b of the free space optics 260 a, 260 b and/or displacement distances 264 a, 264 b of the return beam 291′ so the return beam 291′ is focused in the tip of the receiving waveguides 225 a′, 225 b′. In an implementation, for the system 200″ of FIG. 2F equation 5 expresses the relationship between the separation 221, the rotation speed of the polygon scanner 244 and the target range:

$\begin{matrix} {y = {{focal}\mspace{14mu}{length}*{rotation}\mspace{14mu}{rate}*\frac{4*{range}}{c}}} & (5) \end{matrix}$ where y is the separation 221, focal length is the focal length of the collimation optic 229 (in units of meters); rotation rate is the rotation speed of the polygon scanner 244 (in units of radians per second), c is the speed of light (in units of meters per second) and range is the target range (in units of meters).

In an implementation of the system 200′″, 200″″, a walk-off angle (a) for the free space optic 260 (e.g., a birefringent displacer), defined as the angle (e.g. angle 259 in FIG. 2I) at which the displaced beam (e.g. return beam 291′ with second linear polarization 254) is deflected, is defined by:

$\begin{matrix} {{\tan(a)} = {\left( {1 - \frac{n_{o}^{2}}{n_{e}^{2}}} \right)*\frac{\tan(\theta)}{\left( {1 + {\frac{n_{o}^{2}}{n_{e}^{2}}*{\tan^{2}(\theta)}}} \right)}}} & (6) \end{matrix}$ Where n_(o) and n_(e) are the ordinary and extraordinary refractive index of the free space optic 260 (e.g., a birefringent displacer) and θ is the crystal axis angle (e.g. typically about 45 degrees to maximize the walk-off). In an implementation, the displacement distance 264, is then simply: d=L*tan(α)  (7) Where d is the displacement distance and L is the length 262 of the free space optic 260. In an implementation, the separation 221 between the waveguides in the system 200″′, 200″″ is a sum of the displacement vector d (e.g. using equation 7) and the amount of round-trip delay based separation (e.g. using equation 5). Here, “ordinary refractive index” means an index of refraction experienced by beams with a linear polarization (e.g. linear polarization 254 in FIGS. 2I-2J) orthogonal to the crystal axis and which behave according to the normal law of refraction (e.g. incident normal to the surface of the free space optic 260 and continue in that direction through the free space optic 260). Additionally, “extraordinary refractive index” means an index of refraction experienced by beams with a linear polarization (e.g. linear polarization 252 in FIGS. 2I-2J) parallel to the crystal axis and that do not behave according to the normal law of refraction (e.g. incident normal to the surface of the free space optic 260 and do not continue in that direction through the free space optic 260).

In an implementation, the receiving waveguide 225 a has a separation 221 a that is about 2-5 times the diameter w_(o) of the transmission waveguide 223 and is used to receive return beams 291′ from the target positioned at a further range (e.g. greater than about 50 m). For a target positioned at a further range, the round trip time is longer, the facet 245 a rotates a greater extent than depicted in FIG. 2F and hence the return beam 291′ is deflected by a greater angle 228 to the collimation optic 229. However, for a target positioned at a further range, a diameter of the image 418 (FIG. 4E) of the return beam 291′ on the bistatic transceiver 215 is smaller and thus the separation 221 a is smaller in magnitude and in range (e.g. increased precision) in order to ensure that the image 418 is shifted by the appropriate separation 221 a to the receiving waveguide 225 a. In one implementation, the separation 221 is based on a ratio (e.g. less than one) of the diameter of the image 418. In some implementations, the separation 221 (FIG. 2I) and/or the separations 221 a, 221 b (FIG. 2J) are also based on a ratio (e.g. less than one) of the diameter of the image 418 on the respective bistatic transceivers 215′, 215″ and/or other parameters such as the length of the birefringent displacers and/or the displacement distance of the beam by the birefringent displacers.

In an implementation, the receiving waveguide 225 b has a separation 221 b that is about 5-10 times the diameter w_(o) of the transmission waveguide 223 and is used to receive return beams 291′ from the target positioned at a smaller range (e.g. less than about 50 m). For a target positioned at a smaller range, the round trip time is shorter, the facet 245 a rotates a lesser extent than depicted in FIG. 2F and hence the return beam 291′ is deflected by a lesser angle 228 to the collimation optic 229. However, for a target positioned at a smaller range, a diameter of the image 418 (FIG. 4E) of the return beam 291′ on the bistatic transceiver 215 is larger and thus the separation 221 b is larger in magnitude and in range (e.g. reduced precision) since there is wider latitude on whether the larger image 418 be shifted a particular amount in order to achieve a minimum SNR in the receiving waveguide 225 b.

Thus, in an implementation, the receiving waveguides 225 a, 225 b can be used to receive return beams 291′ from targets at different ranges as the beam 205″ is scanned over the angle range 227 at a fixed rotation speed. In an example implementation, the waveguide 225 a receives return beams 291′ from targets positioned at a longer range over a first portion of the angle range 227 and the waveguide 225 b receives return beams 291′ from targets positioned at a shorter range over a second portion of the angle range 227. However, in other implementations, only one receiving waveguide 225 a or 225 b is used to receive return beams 291′ from targets within one value or within a range of values of the target range as the beam 205″ is scanned over the angle range 227 (e.g. from about 0 m to about 250 m).

In an implementation, the system 200″ excludes the free space optic 226 and hence the receiving waveguide 225 a is provided along the receiving path 224 and connected to the optical mixer 284. The reference beam 207 b is transmitted in a waveguide along the LO path 220 so that the reference beam 207 b is combined with the return beam 291′ from the receiving waveguide 225 a in the optical mixer 284. In an implementation, where multiple receiving waveguides 225 a, 225 b are provided, a similar arrangement is provided for the receiving waveguide 225 b so that the receiving waveguide 225 b is provided along the receiving path 224 and connected to a respective optical mixer 284 where a respective reference beam 207 b is combined with the return beam 291′ from the receiving waveguide 225 b. In one implementation, where one receiving waveguide 225 a is in the bistatic transceiver 215, only one processing channel (e.g. one receiving waveguide, one optical mixer, one waveguide along the LO path) is provided. In an example implementation, only one processing channel is provided for the receiving waveguide 225′ of the bistatic transceiver 215′ of the system 200′″ of FIG. 2I. However, multiple processing channels are provided for the receiving waveguides 225 a′, 225 b′ of the system 200′″ of FIG. 2K, which depicts a linear array of waveguides (e.g. one dimensional linear array) including more than one pair of waveguides, where each pair of waveguides includes the transmission waveguide 223′ and the receiving waveguide 225′ of FIG. 2I. In another implementation, where multiple receiving waveguides 225 a, 225 b are provided, multiple processing channels are provided. In an example implementation, multiple processing channels are provided for the receiving waveguides 225 a′, 225 b′ of the bistatic transceiver 215″ of the system 200″″ of FIG. 2J. In this example implementation, the multiple processing channels are configured to process the orthogonal linear polarization of the multiple return beams received in the tips of the receiving waveguides 225 a′, 225 b′. In another example implementation, the bistatic transceiver 215″ is a linear array (e.g. one dimensional array) and thus multiple processing channels are provided to simultaneously process multiple return beam signals (e.g. with orthogonal linear polarization) received in multiple receiving waveguides in a one-dimensional linear array of waveguides. Thus, in an implementation, the systems 200″, 200′″, 200″″ include an equivalent number of processing channels as the number of receiving waveguides 225. In an implementation, the acquisition system 240 and/or processing system 250 is configured to process the return beam 291′ at sequential time periods from the receiving waveguides 225 (e.g. process return beam 291′ from receiving waveguide 225 a over a first time period, process return beam 291′ from receiving waveguide 225 b over a second time period) since the return beams 291′ are sequentially received at non-overlapping time periods from the receiving waveguides 225 a, 225 b. The acquisition system 240 and/or processing system 250 are configured to simultaneously process the multiple return beams received in the systems 200′″, 200″″.

4. Coherent LIDAR System Parameters

In an implementation, monostatic coherent LIDAR performance of the system 200′ is modeled by including system parameters in a so called “link budget”. A link budget estimates the expected value of the signal to noise ratio (SNR) for various system and target parameters. In one implementation, on the system side, a link budget includes one or more of output optical power, integration time, detector characteristics, insertion losses in waveguide connections, mode overlap between the imaged spot and the monostatic collection waveguide, and optical transceiver characteristics. In another implementation, on the target side, a link budget includes one or more of atmospheric characteristics, target reflectivity, and target range.

FIG. 4A is a graph that illustrates an example signal-to-noise ratio (SNR) versus target range for the return beam 291 in the system 200′ of FIG. 2D without scanning, according to an implementation. In other implementations, FIG. 4A depicts an example of SNR versus target range for the return beam 291 in the system 200 of FIG. 2A. The horizontal axis 402 is target range in units of meters (m). The vertical axis 404 is SNR in units of decibels (dB). A curve 410 depicts the values of SNR versus range that is divided into a near field 406 and a far field 408 with a transition from the near field 406 of the curve 410 with a relatively flat slope to the far field 408 of the curve 410 with a negative slope (e.g. about −20 dB per 10 m). The reduction in SNR in the far field 408 is dominated by “r-squared” losses, since the scattering atmosphere through which the return beam 291 passes grows with the square of the range to the target while the surface area of the optical waveguide tip 217 to collect the return beam 291 is fixed. FIG. 4B is a graph that illustrates an example of a curve 411 indicating 1/r-squared loss that drives the shape of the SNR curve 410 in the far field 408, according to an implementation. The horizontal axis 402 is range in units of meters (m) and the vertical axis 407 is power loss that is unitless.

In the near field 406, a primary driver of the SNR is a diameter of the collimated return beam 291 before it is focused by the collimation optics 229 to the tip 217. FIG. 4C is a graph that illustrates an example of collimated beam diameter versus range for the return beam 291 in the system 200′ of FIG. 2D without scanning, according to an implementation. The horizontal axis 402 is target range in units of meters (m) and the vertical axis 405 is diameter of the return beam 291 in units of meters (m). In an implementation, curve 414 depicts the diameter of the collimated return beam 291 incident on the collimation optics 229 prior to the return beam 291 being focused to the tip 217 of the optical waveguide. The curve 414 illustrates that the diameter of the collimated return beam 291 incident on the collimation optics 229 increases with increasing target range.

In an implementation, in the near field 406, as the diameter of the collimated return beam 291 grows at larger target ranges, a diameter of the focused return beam 291 by the collimation optics 229 at the tip 217 shrinks. FIG. 4D is a graph that illustrates an example of SNR associated with collection efficiency of the return beam 291 at the tip 217 versus range for the transmitted signal in the system of FIG. 2D without scanning, according to an implementation. The horizontal axis 402 is target range in units of meters (m) and the vertical axis 404 is SNR in units of decibels (dB). The curve 416 depicts the near field SNR of the focused return beam 291 by the collimation optics 229 at the tip 217 based on target range. At close ranges within the near field 406, an image 418 a of the focused return beam 291 at the tip 217 by the collimation optics 229 is sufficiently larger than the core size of the single mode optical fiber tip 217. Thus the SNR associated with the collection efficiency is relatively low. At longer ranges within the near field 406, an image 418 b of the focused return beam 291 at the tip 217 by the collimation optics 229 is much smaller than the image 418 a and thus the SNR attributable to the collection efficiency increases at longer ranges. In an implementation, the curve 416 demonstrates that the SNR in near field 406 has a positive slope (e.g. +20 dB per 10 meters) based on the improved collection efficiency of the focused return beam 291 at longer ranges. In one implementation, this positive slope in the near field SNR cancels the negative slope in the near field SNR discussed in FIG. 4B that is attributable to “r-squared” losses and thus leads to the relatively flat region of the SNR curve 410 in the near field 406. The positive slope in the SNR curve 416 in FIG. 4D does not extend into the far field 408 and thus the “r-squared” losses of FIG. 4B dominate the far field 408 SNR as depicted in the SNR curve 410 in the far field 408.

While the discussion in relation to FIGS. 4A-4D predicts SNR of the return beam 291 as a function of the target range, the predicted SNR in FIGS. 4A-4D is for a monostatic coherent LIDAR system 200′ and does not fully characterize the performance of the scanned monostatic coherent LIDAR system 200′ since it does not consider a scan rate of the scanning optics 218. FIGS. 4E-4G below are discussed in the context of the bistatic coherent LIDAR system 200″ where the beam 205″ is scanned at a scan rate that is greater than zero. In an implementation, due to round trip delay of the return beam 291′, the receive mode of the return beam 291′ will laterally shift or “walk off” from the transmitted mode of the transmitted beam 205′ when the beam is being scanned by the scanning optics 218 (e.g. polygon scanner 244). In an implementation, the return beam 291′ is deflected at the angle 228 relative to the transmitted beam 205′ and the collimation optic 229 focuses the return beam 291′ into the tip 217 of the receiving waveguide 225 a if the lateral shift or “walk off” corresponds to the separation 221 a or is within a threshold of the separation 221 a. In an implementation, the threshold is a maximum ratio of a diameter of an image of the return beam 291′ on the tip 217 of the receiving waveguide 225 (e.g. ratio less than 1). In one implementation, the walkoff needs to be such that the overlap of the image 418 of the return beam 291′ with the receiving waveguide 225 leads to optimal collection efficiency, e.g. the walkoff is such that the center of the image 418 is within ±10% of the tip 217 of the receiving waveguide 225. FIG. 4E is an image that illustrates an example of beam walkoff for various target ranges and scan speeds in the system 200″ of FIG. 2E, according to an implementation. The horizontal axis 402 is target range and the vertical axis 422 is scan speed of the beam using the scanning optics 218. As FIG. 4E depicts, there is no beam walkoff when the beam is not scanned (bottom row) since the image 418 a of the focused return beam 291′ is centered on the tip 217 of the transmission waveguide 223 demonstrating no beam walkoff at short target range and the image 418 b of the focused return beam 291′ is also centered on the tip 217 of the transmission waveguide 223 demonstrating no beam walkoff at far target range. In an implementation, since the beam 291′ is not centered on or near the tip 217 of the receiving waveguide 225 and/or there is little or no beam walkoff and thus the beam walkoff is not within a threshold of the separation 221 between the transmission waveguide 223 and the receiving waveguide 225. Consequently, this is not an optimal arrangement for the bistatic transceiver system 200″. As depicted in FIG. 4E, a diameter of the image 418 a is greater than the separation 221 and thus the image 418 a of the return beam 291′ partially overlaps with the tip 217 of the receiving waveguide 225. Consequently some portion of the return beam 291′ is received in the tip 217 of the receiving waveguide 225 at the shorter target range and thus the signal to noise ratio (SNR) is greater than zero, even when the beam 205″ is not scanned. Additionally, as depicted in FIG. 4E, a diameter of the image 418 b is less than or about equal to the separation 221 and thus the image 418 b of the return beam 291′ may not overlap with the tip 217 of the receiving waveguide 225 for longer target range. As a result, little or no portion of the return beam 291′ is received in the tip of the receiving waveguide 225 at longer target range when the beam 205″ is not scanned.

When the beam 205″ is scanned at a moderate scan speed (middle row in FIG. 4E), a moderate beam walkoff 419 a is observed between the image 418 a of the focused return beam 291′ and the tip 217 of the transmission waveguide 223 for small target range and a larger beam walkoff 419 b is observed between the image 418 b of the focused return beam 291′ and the tip 217 of the transmission waveguide 223 for far target range. Although the beam walkoff 419 b is greater for the larger target range than the beam walkoff 419 a for the smaller target range, the return beam 291′ has higher SNR at smaller target range due to the much larger diameter of the image 418 a on the receiving waveguide 225. Since the walkoff 419 b is less than a ratio of a diameter of the image 418 b, this is not optimal arrangement for the bistatic transceiver system 200″ for far target range. However, in some implementations, the spacing 221 is selected based on the walkoff 419 a so that the receiving waveguide 225 a is configured to receive the return beam 291′ at the short range when the polygon scanner 244 is rotating at the moderate scan speed, since the increased diameter of the image 418 a for short target range may result in the SNR of the return beam 291′ being greater than a SNR threshold, despite the small walkoff 419 a.

When the beam 205″ is scanned at a high scan speed (top row in FIG. 4E), a beam walkoff 421 a is observed at short range that exceeds the beam walkoff 419 a at the moderate scan speed and a beam walkoff 421 b is observed at large range that exceeds the beam walk off 419 b at the moderate scan speed. Thus, the beam walkoff increases as the target range and scan speed increase. In an implementation, increased target range induces a time delay during which the image 418 a, 418 b shifts away from the tip 217 of the transmission waveguide 223. Thus a model of the mode overlap accounts this walkoff appropriately. In one implementation, such a model should limit the beam walkoff 419, 421 based on a diameter of the image 418 (e.g. no greater than half of the diameter of the image 418) and thus for the target 418 a at smaller target range there is wider latitude for the acceptable range of the beam walkoff 419, 421. In one implementation, the spacing 221 b is adjusted based on the beam walkoff 421 a and the spacing 221 a is adjusted based on the beam walkoff 421 b so that the polygon scanner 244 can be set at a fixed optimal scan speed and so that return beams 291′ from a target at a shorter range is deflected into the receiving waveguide 225 b and return beams 291′ from a target at a longer range is deflected into the receiving waveguide 225 a. In this example implementation, the beam walkoff 421 a is within the threshold of the spacing 221 a and the beam walkoff 421 b is within the threshold of the spacing 221 b. In some implementations, beam walkoff similar to that depicted in FIG. 4E can be generated for various target ranges, scan speeds, birefringent displacer lengths and/or displaced beam distances in the system 200′″ of FIG. 2I and/or the system 200″″ of FIG. 2J.

FIG. 4F is a graph that illustrates an example of coupling efficiency versus target range for various scan rates in the system 200″ of FIG. 2E, according to an implementation. The horizontal axis 402 is target range in units of meters (m) and the vertical axis 430 is coupling efficiency which is unitless. In an implementation, the coupling efficiency is inversely proportional to the difference between the separation 221 and the beam walkoff 419, 421 and/or the diameter of the image 418 (e.g. for larger diameter, wider latitude in the difference between the separation 221 and beam walkoff 419, 421 and for smaller diameter, narrower latitude in the difference). A first curve 432 a depicts the coupling efficiency of the focused return beam 291 into the fiber tip 217 in the monostatic system 200′ for various target ranges based on no scanning of the beam 205′. The coupling efficiency remains relatively high and constant for a wide range of target ranges. A second curve 432 c depicts the coupling efficiency of the focused return beam 291′ into the tip 217 of the receiving waveguide 225 for various target ranges based on moderate scan rate of the beam. In an implementation, the coupling efficiency at the moderate scan rate peaks at a high target range (e.g. about 450 m) and then decreases for target ranges above and below this high target range. A third curve 432 b depicts the coupling efficiency of the focused return beam 291′ into the tip 217 of the receiving waveguide 225 for various target ranges based on a high scan rate of the beam. In an implementation, the coupling efficiency of the high scan rate peaks at a moderate target range (e.g. about 180 m) and then decreases as target range increases. A fourth curve 432 d depicts the coupling efficiency of the focused return beam 291′ into the tip 217 of the receiving waveguide 225 for various target ranges based on no scanning of the beam. Since no scanning of the beam results in the return beam 291′ being centered on the transmission waveguide 223 (bottom row of FIG. 4E), the coupling efficiency is about 0 throughout the target range. Consequently, no scanning of the beam 205″ is not an advantageous mode of operation for the bistatic LIDAR system 200″. In some implementations, coupling efficiency similar to that depicted in FIG. 4F can be generated for various target ranges, scan speeds, birefringent displacer lengths and/or displaced beam distances in the system 200′″ of FIG. 2I and/or the system 200″″ of FIG. 2J.

Based on the curves in FIG. 4F, no scanning results in little to no coupling efficiency into the receiving waveguides 225 and thus is not optimal for the bistatic LIDAR system 200″. Also, scanning too slow makes it impossible to see within a wide target range (e.g. <300 m). In this instance, the beam walkoff 419 b of the image 418 b of the focused return beam 291′ only approaches the separation 221 at very large target range (e.g. beyond 300 m). Consequently, it is not optimal to operate the bistatic LIDAR system 200″ at the slow scan speed, at least to capture return beam 291′ data for targets having a range shorter than this very large range (e.g. for targets with range <300 m). FIG. 4F also depicts that scanning at an optimal speed (e.g. curve 432 b) makes it possible to see targets positioned in a wide target range (e.g. from about 100 m to about 300 m). This is based on the beam walkoff 421 b being within the threshold of the separation 221. In an example implementation, the moderate scan speed is in a range from about 1000 deg/sec to about 2000 deg/sec and the optimal scan speed is in a range from about 4000 deg/sec to about 7000 deg/sec.

FIG. 4G is a graph that illustrates an example of SNR versus target range for various scan rates in the system 200″ of FIG. 2E, according to an implementation. The horizontal axis 402 is target range in units of meters (m) and the vertical axis 404 is SNR in units of decibels (dB). A first curve 440 d depicts the SNR of the focused return beam 291′ on the tip 217 of the receiving waveguide 225 based on target range where the beam is not scanned. Although there is no beam walkoff when the beam is not scanned, the image 418 a of the focused return beam 291′ partially overlaps with the tip 217 of the receiving waveguide 225 (bottom row of FIG. 4E), and hence the SNR is greater than zero and may be greater than the SNR threshold 442 since the diameter of the image 418 a is relatively large. Additionally, when the beam is not scanned (bottom row of FIG. 4E) for large target range the diameter of the image 418 b of the focused return beam 291′ is much smaller than at smaller target range and fails to overlap with the tip 217 of the receiving waveguide 225. Hence, beyond some target range (e.g. about 90 m) the SNR of the curve 440 d approaches zero.

A second curve 440 b depicts the SNR of the focused return beam 291′ on the tip 217 of the receiving waveguide 225 based on target range where the beam is scanned at a slow scan rate. In an example implementation, the slow scan rate is about 2500 degrees per sec (deg/sec) or in a range from about 1000 deg/sec to about 4000 deg/sec or in a range from about 500 deg/sec to about 5000 deg/sec. A third curve 440 c depicts the SNR of the focused return beam 291′ on the tip 217 of the receiving waveguide 225 based on target range where the beam is scanned at an optimal scan rate. In an example implementation, the optimal scan rate is about 5500 deg/sec or in a range from about 4000 deg/sec to about 7000 deg/sec or in a range from about 3000 deg/sec to about 8000 deg/sec. In an implementation, the slow scan rate and optimal scan rate are based on one or more parameters of the system 200″ including a beam size and/or the separation 221 and/or a goal of the system 200″. In an example implementation, the numerical ranges of the slow scan rate and optimal scan rate above are based on a collimated beam with a diameter of about 1 centimeter (cm) used to scan an image out to a maximum target range of about 400 meters (m).

Ultimately, a difference between the beam walk off 419, 421 and the separation 221 is a significant inhibitor of SNR in coherent bistatic LIDAR system 200″ and/or a diameter of the image 418 for a particular target range indicates a tolerance or precision of the difference in order to achieve a threshold SNR. In an implementation, the scan rate of the beam in the bistatic system 200″ is set at a fixed scan rate (e.g. fixed speed of the angular velocity 249 of the polygon scanner 244) over the angle range and the resulting target range, where the fixed scan rate is chosen so that the associated SNR of the fixed scan rate is above an SNR threshold over the target range. As previously discussed, the SNR curves of FIG. 4G can be generated for the return beams received at the tips of the receiving waveguides in the systems 200′″ (FIG. 2I) and system 200″″ (FIG. 2J). In an implementation, similar methods as discussed herein can be employed with the SNR curves to determine the fixed scan rate of the scanning optics in those systems (e.g. set at a scan rate where the associated SNR curve is above an SNR threshold over the target range). In conventional coherent LIDAR systems, this results in a relatively low fixed scan rate being used to scan the beam over a scan trajectory 460, which results in large gaps 462 between adjacent scans, as depicted in FIG. 4H. The scan speed limitation leads to dense sampling along the beam trajectory 460. When the beam is scanned over a reasonably large field of few (e.g. 10's of degrees in either dimension), the beam trajectory 460 leaves large gaps 462 in the angular coverage. This is not ideal as targets positioned in the large gaps 462 go undetected. A “square grid” of rectangular sampling is not achieved. Instead, an asymmetry is observed between the sampling along the scan trajectory 460 and the gaps 462 between the trajectory 460 which can be greater than 10:1. With this problem in mind, the inventors of the present disclosure developed several complementary solutions including one or more of maximizing the fixed beam scan speed and generate efficient hardware solutions (e.g. polygon scanner 244) for these concepts.

In addition to the scan rate of the beam, the SNR of the return beam 291′ is affected by the integration time over which the acquisition system 240 and/or processing system 250 samples and processes the return beam 291′. In some implementations, the beam is scanned between discrete angles and is held stationary or almost stationary at discrete angles in the angle range 227 for a respective integration time at each discrete angle. In other implementations, the beam is scanned at a fixed scan rate (e.g. using the polygon scanner 244) throughout the angle range 227. The SNR of the return beam 291′ is affected by the value of the integration time and/or the target range and/or the scan rate and/or the separation 221. Accordingly, a longer integration time is needed to achieve the same SNR for a return beam 291′ from a longer target range.

FIG. 4I is a graph that illustrates an example of SNR versus target range for various integration times in the system 200″ of FIG. 2E, according to an implementation. The horizontal axis 402 is target range in units of meters (m) and the vertical axis 404 is SNR in units of decibels (dB). A first curve 450 a depicts SNR values of the return beam 291′ over the target range, where the system 200″ is set to a first integration time (e.g. 3.2 μs). A second curve 450 b depicts SNR values of the return beam 291′ over the target range, where the system 200″ is set to a second integration time (e.g. 1.6 μs). A third curve 450 c depicts SNR values of the return beam 291′ over the target range, where the system 200″ is set to a third integration time (e.g. 800 ns). A fourth curve 450 d depicts SNR values of the return beam 291′ over the target range, where the system 200″ is set to a fourth integration time (e.g. 400 ns). The curves 450 demonstrate that for a fixed target range, an increased SNR is achieved with increasing integration time. The curves 450 also demonstrate that for a fixed integration time, the SNR of the return beam 291′ decreases with increased range for the reasons previously discussed. In one implementation, a fixed integration time is selected (e.g. 1.6 μs) for the scanning at the range of angles 227 and resulting target ranges, so that the SNR associated with the fixed integration time exceeds an SNR threshold 452 over the target range. Another implementation involves minimizing the integration time at each angle within the range of angles 227 using the target range at each angle, so to minimize the integration time over the range of angles 227. FIG. 4J is a graph that illustrates an example of a measurement rate versus target range in the system 200″ of FIG. 2E, according to an implementation. The horizontal axis 402 is target range in units of meters (m) and the vertical axis 474 is number of allowable measurements per unit time in units of number of allowable measurements per second. Curve 476 depicts the number of allowable measurements per second at each target range. In an implementation, curve 476 represents an inverse of the integration time, e.g. the number of return beams 291′ that can be detected at each target range per second whereas integration time conveys how long it takes to process the return beam 291′ at each target range. Curve 478 is also provided and is a good target of the number of allowable measurements per second at each target range. The curve 478 is based on power of 2 intervals for a given ADC (analog to digital conversion) rate. Curve 478 represents a good target of the number of allowable measurements per second since when the number of digitized samples is a power of 2, the fast Fourier transform on such a length signal is more efficient. The curves 450 change based on one or more system 200″ system parameters including but not limited to the waveguide separation 221, the power of the transmitted signal 205′ and the focal length of the collimation optic 229. In an implementation, curves similar to the curves 450 of FIG. 4I can be generated for the integration time of the return beams received at the tips of the receiving waveguides in the systems 200′″ (FIG. 2I) and system 200″″ (FIG. 2J). Similar methods as discussed herein can be employed to determine the fixed integration time, e.g. the value of the integration time of the curve which exceeds the SNR threshold over the target range.

5. Free Space Optic for Bistatic LIDAR System

FIGS. 2G and 2H are block diagrams that illustrates a top view of example components of a monostatic LIDAR system, according to an implementation. The monostatic LIDAR systems depicted in FIGS. 2G and 2H are similar to the system of FIG. 2D with the exception of the features discussed herein. Unlike the system of FIG. 2D, the transmission path 222′ and receiving path 224′ of the systems 207, 209 of FIGS. 2G and 2H are polarization maintaining (PM) waveguides with orthogonal orientation. Additionally, the monostatic LIDAR system 207 of FIG. 2G includes a free space optic 226′ (instead of the free space optic 226 of FIG. 2D) and a quarter wave plate 211 positioned between the collimating optics 229 and the target 292. In some implementations, the free space optic 226′ may be an optical isolator or a polarization beam splitter/combiner but is not limited thereto. The free space optic 226′ (e.g., a polarization beam splitter/combiner) polarizes the beam from the transmission path 222′ with a first linear polarization (e.g. vertical linear polarization) which is then output from the tip 217 of the monostatic fiber. The collimating optic 229 then collimates the beam with the first linear polarization after which the quarter wave plate 211 converts the first linear polarization of the beam into a first circular polarization (e.g. right circular polarization). The beam then reflects from the target 292 which causes a phase change in the first circular polarization to a second circular polarization (e.g. left circular polarization) that is opposite to the direction of the first circular polarization. The collimating optic 229 then focuses this return beam into the tip 217 of the monostatic fiber. The free space optic 226′ then polarizes the beam by converting the second circular polarization into a second linear polarization (e.g. horizontal linear polarization) that is orthogonal to the first linear polarization of the transmit beam. The PM fiber of the receiving path 224′ is aligned with the second linear polarization so the return beam with the second linear polarization is only received in the PM fiber of the receiving path 224′ and is not received in the PM fiber of the transmission path 222′ since it is aligned orthogonal to the second linear polarization.

The system 209 of FIG. 2H operates in a similar manner as the system 207 of FIG. 2G with the exception that the system 209 of FIG. 2H features a free space optic 226 and excludes the quarter wave plate 211. In some implementations, the free space optic 226 in FIG. 2H may be a fiber coupled circulator but is not limited thereto. In some implementations, the free space optic 226 in FIG. 2H (e.g., a fiber coupled circulator) is designed to only permit travel in one direction from a first port (e.g. transmission path 222′ waveguide) to a second port (e.g. waveguide tip 217) and not to permit travel in the opposite direction from the second port to the first port. Similarly, the free space optic₂₂₆ (e.g., a fiber coupled circulator) is designed to only permit travel in one direction from the second port (e.g. the waveguide tip 217) to a third port (e.g. receiving path 224′ waveguide) and to not permit travel in the opposite direction from the third port to the second port. This ensures that the transmit beam from the transmission path 222′ waveguide is only transmitted from the waveguide tip 217 and that the return beam received at the waveguide tip 217 is only received at the receiving path 224′ waveguide.

The inventors of the present disclosure noticed that the systems of FIGS. 2G and 2H are specifically designed for monostatic LIDAR systems, where the transmit and receive modes are separated within the monostatic fiber. Thus, these systems do not provide a mechanism for spatial separation of the transmit and receive modes in a bistatic LIDAR system with spatially separated transmission and receiving waveguides in a fiber array or bistatic transceiver. Additionally, the inventors of the present disclosure noticed that the system of FIG. 2G requires placement of polarization transforming optics (e.g. quarter wave plate 211) beyond the collimating optics 229 and thus recognized that it would be more advantageous to design an arrangement which conveniently employed free space optics more proximate to the LIDAR system. For example, placement of free space optics more proximate to the LIDAR system would advantageously permit easier maintenance of critical alignment of the transmit and receive modes relative during operation of the bistatic LIDAR system. In other examples, advantages of the placement of the free space optics more proximate to the LIDAR system include using smaller (e.g. less expensive) components as the beam is much smaller near the waveguide tips relative to the collimated beam. Also, use of circulators on the other side of the collimating optics would have to use birefringent wedges to create an angular offset of the receive mode which would be more difficult to maintain the alignment tolerance.

FIG. 2I is a block diagram that illustrates a top view of example components of a bistatic LIDAR system 200′″, according to an implementation. In an implementation, the bistatic LIDAR system 200′″ is similar to the bistatic LIDAR system 200″ with the exception of the features discussed herein. In an implementation, the system 200′″ includes a fiber array or bistatic transceiver 215′ that includes a pair of waveguides including a transmission waveguide 223′ and a receiving waveguide 225′ spaced apart from the transmission waveguide guide 223′ by a separation 221. In an example implementation, the separation 221 is about 127 μm or in a range from about 100 μm to about 150 μm and/or in a range from about 50 μm to about 300 μm. In one implementation, the bistatic transceiver 215′ is a linear array (e.g. one dimensional or two dimensional) of waveguides. In an example implementation, the bistatic transceiver 215′ are standard telecom components which have sub-micron alignment tolerances between parallel optical fibers defined through photolithographic techniques. In one implementation, the transmission waveguide 223′ and/or the receiving waveguide 225′ are polarization maintaining (PM) waveguides or fibers. In an example implementation, the transmission waveguide 223′ and the receiving waveguide 225′ are PM fibers with orthogonal orientations (e.g. transmission waveguide 223′ is PM fiber with vertical orientation and receiving waveguide 225′ is PM fiber with horizontal orientation or vice versa). In this implementation, the transmitted beam 205′ from the tip of the transmission waveguide 223′ is polarized, such as with a first linear polarization 252 (e.g. vertical linear polarization) corresponding to the PM fiber of the transmission waveguide 223′. Additionally, in this implementation, the return beam 291′ received at the tip of the receiving waveguide 225′ is polarized, such as with a second linear polarization 254 (e.g. horizontal linear polarization) that corresponds to the PM fiber of the receiving waveguide 225′ and is orthogonal to the first linear polarization.

In an implementation, the bistatic LIDAR system 200′″ includes a free space optic 260 (e.g., a birefringent displacer) that does not displace beams with a first linear polarization 252 (e.g. vertical linear polarization) and does displace beams with a second linear polarization 254 (e.g. horizontal linear polarization) orthogonal to the first linear polarization by a distance 264 orthogonal to a direction of the beams. In an implementation, the distance 264 is based on a length 262 of the free space optic 260 that the beam traverses and/or the material of the free space optic 260. In an example implementation, the free space optic 260 is made from any birefringent material such as Calcite, YVO₄, αBBO or Rutile. In another example implementation, the free space optic 260 has precision fabrication tolerances of about ±0.01 millimeters (mm) on the length 262 and/or ±0.1 degrees for a crystal axis and surface angles. This permits micron-level tolerances on the displacement distance 264. For purposes of this description, “surface angles” refers to any tilt angle of the input and output surface of the birefringent displacer to incident and outgoing beams. Additionally, for purposes of this description, the “crystal axis” is a c-axis (e.g. optic axis) of the crystal of the birefringent displacer that defines the direction of the displacement (e.g. displacement 264 in FIG. 2I) and its angle (e.g. angle 259 in FIG. 2I) relative to the beam is used to calculate the walk-off angle. In an example implementation, the angle in the plane of displacement (e.g. angle 259 in the plane of FIG. 2I) in one example is about 45 degrees relative to direction of the beam (e.g. direction of the return beam 291′ incident on the free space optic 260 in FIG. 2I). The direction of the crystal axis depends on the material, since the type of material indicates positive or negative birefringence. In the example implementation of FIG. 2I, the free space optic 260 is made from YVO4 material (e.g. positive birefringence) and would have an optic axis in the plane of FIG. 2I, angled at about 45 degrees from lower left to upper right (not directional). In another example implementation, a displacer made from aBBO material (e.g. negative birefringence) would have an optic axis angled from top left to bottom right in the plane of FIG. 2I. The angle of deflection (e.g. walk-off angle) is defined in the equation 6.

In an implementation, the transmitted beam 205′ from the transmission waveguide 223′ with the first linear polarization 252 is incident on the free space optic 260. Since the free space optic 260 does not displace incident beams with the first linear polarization 252, the transmitted beam 205′ passes through free space optic 260 without undergoing displacement. The transmitted beam 205′ is then incident on a polarization transforming optic to adjust a relative phase between orthogonal field components of the transmitted beam 205′. In an implementation, the polarization transforming optic is a quarter wave plate 211 which adjusts the first linear polarization 252 of the transmitted beam 205′ into a first circular polarization 256 having a first direction (e.g. clockwise). In an example implementation, the quarter wave plate is made of quartz and oriented at about 45 degrees relative to the axis of displacement of the free space optic 260.

The transmitted beam 205′ with the first circular polarization 256 is then collimated by the collimating optics 229 and/or scanned by the scanning optics 218. In some implementations, the scanning optics 218 is omitted and the transmitted beam 205′ is collimated by the collimating optics 229. In an example implementation, the collimating optics 229 is an aspheric lens that may be made by grinding and polishing or by molding. In another example implementation, the collimating optics 229 is a singlet or multi-element (e.g. doublet, triplet, etc.) spherical lens. As depicted in FIG. 2I, the free space optic 260 (e.g., birefringent displacer) and polarization transforming optic (e.g. quarter wave plate 211) are positioned between the bistatic transceiver 215′ and the collimating optics 229, which is distinct from the monostatic system 207 of FIG. 2G where the quarter wave plate 211 is positioned on a far side of the collimating optics 229 from the monostatic waveguide tip 217.

In an implementation, the collimating optics 229 and/or scanning optics 218 direct the transmitted beam 205′ at the target 292. Here, reflection of the return beam 291′ from the target 292 causes a phase change from the first circular polarization 256 to a second circular polarization 258 with a second direction (e.g. counter clockwise) opposite to the first direction of the first circular polarization 256. The collimating optics 229 and/or scanning optics 218 direct the return beam 291′ with the second circular polarization 258 at the quarter wave plate 211 which adjusts the second circular polarization 258 into the second linear polarization 254 that is orthogonal to the first linear polarization 252 of the transmitted beam 205′.

In an implementation, the return beam 291′ with the second linear polarization 254 is incident on the free space optic 260 (e.g., a birefringent displacer). Since the free space optic 260 displaces incident beams with the second linear polarization 254, the return beam 291′ is displaced by the distance 264 in a direction orthogonal to a direction of travel of the return beam 291′ as the return beam 291′ passes through the free space optic 260. In an implementation, the length 262 of the free space optic 260 and/or the material of the free space optic 260 is selected so that the distance 264 is adjusted based on the separation 221 of the transmission waveguide 223′ and receiving waveguide 225′. In an example implementation, the length 262 and/or material of the free space optic 260 is selected so that the distance 264 is about equal to (e.g. within ±2% or ±5%) the separation 221. In another example implementation, the material of the free space optic 260 is selected to be Yttrium orthovanadate (YV0₄) and of crystal orientation and the length 262 is sized so that the distance 264 is about equal to the separation 221.

As depicted in FIG. 2I, after the return beam 291′ is displaced by the distance 264, the return beam 291′ is aligned with and is received at the tip of the receiving waveguide 225′. In an example implementation, typical coupling losses of less than about 1.2 decibels (dB) can be achieved. In an implementation, the return beam 291′ is then combined with a reference beam 207 b (or LO signal) using one or more optical mixers 284 in the detector array 230. In an implementation, the return beam 291′ advantageously has a linear polarization 254 that is orthogonal to the linear polarization 252 of the transmitted beam 205′ and thus the processing of the return beam 291′ may provide further data pertaining to the target 292 (e.g. using the linear polarization 254 of the return beam 291′) that are not attained in the other bistatic systems. In general, but not exclusively, natural materials tend to depolarize upon scattering, while man-made materials including painted metal tend to maintain polarization better. The polarization properties of the return beam 291′ acquired during the processing steps can be used to distinguish and identify objects from similarly shaped objects. In an example implementation, target identification applications on air and land static and mobile platforms for Defense or commercial can benefit from polarization information.

Although FIG. 2I depicts one implementation where the bistatic transceiver 215′ includes one pair of waveguides including one transmission waveguide 223′ and one receiving waveguide 225′ arranged in an array (e.g. one dimensional array), in other implementations, the bistatic transceiver 215′ includes more than one pair of waveguides. FIG. 2K is a block diagram that illustrates a top view of example components of a bistatic LIDAR system 200′″, according to an implementation. The bistatic transceiver 215′ of FIG. 2K features more than one pair of waveguides arranged in an array (e.g. one dimensional array), including two or more transmission waveguides 223 a′, 223 b′ and two or more receiving waveguides 225 a′, 225 b′, where there is a receiving waveguide 225 a′, 225 b′ adjacent each respective transmission waveguide 223 a′, 225 b′. As depicted in FIG. 2K, in an implementation, transmitted beams 205 a′, 205 b′ are outputted from tips of the transmission waveguides 223 a′, 223 b′, either in parallel or in series. As further depicted in FIG. 2K, in an implementation, return beams 291 a′, 291 b′ are received at tips of the receiving waveguides 225 a′, 225 b′, either in parallel or in series. In an example implementation, the transmitted beams 205 a′, 205 b′ output from the tips of the transmission waveguides 223 a′, 223 b′ form a fan of angularly separated transmit beams and the return beams 291 a′, 291 b′ received at the tips of the receiving waveguides 225 a′, 225 b′ forms a fan of angularly separated return beams. As depicted in FIG. 2K, in an implementation a single free space optic 260 (e.g., a single birefringent displacer) and polarization transforming optic (e.g. quarter wave plate 211) are used to shape multiple beams from the bistatic transceiver 215′, which provides an advantage over conventional bistatic systems where individual shaping optics are utilized for each respective pair of transmission and receiving waveguides.

FIG. 2J is a block diagram that illustrates a top view of example components of a bistatic LIDAR system 200″″, according to an implementation. The system 200″″ is similar to the system 200′″ of FIG. 2I with the exceptions of the features discussed herein. In an implementation, the fiber array or bistatic transceiver 215″ of FIG. 2J includes a group of waveguides (e.g. three) including a transmission waveguide 223, a first receiving waveguide 225 a′ spaced apart from a first side of the transmission waveguide 223 by a first separation 221 a and a second receiving waveguide 225 b′ spaced apart from a second side of the transmission waveguide 223 b by a second separation 221 b. In an implementation, the waveguides of the bistatic transceiver 215″ are arranged in in one-dimensional array or two dimensional array. In an implementation, the two dimensional array is based on a waveguide arrangement consistent with adjacent stacking of the one-dimensional array. In some implementations, the first separation 221 a is about equal to the second separation 221 b. Although FIG. 2J depicts that the bistatic transceiver 215″ includes a pair of receiving waveguides, with a receiving waveguide on opposite sides of the transmission waveguide, in other implementations, the pair of receiving waveguides are positioned on a same side of the transmission waveguide, such as with the arrangement of FIG. 2F. In an example implementation, the separation 221 a and/or 221 b is about 127 μm or in a range from about 100 μm to about 150 μm and/or in a range from about 50 μm to about 300 μm. In one implementation, the bistatic transceiver 215″ is a linear array (e.g. one dimensional or two dimensional) of waveguides. In an example implementation, the bistatic transceiver 215″ are standard telecom components which have sub-micron alignment tolerances between parallel optical fibers defined through photolithographic techniques.

In an implementation, the transmission waveguide 223 of FIG. 2J is a single mode (SM) fiber and the receiving waveguides 225 a′, 225 b′ are polarization maintaining (PM) fibers that are oriented orthogonal with respect to each other. In one implementation, the transmitted beam 205′ from the tip of the transmission waveguide 223 is non-polarized 253 which is accommodated by the SM fiber of the transmission waveguide 223. In another implementation, the first component 237 of the return beam 291′ received in the first receiving waveguide 225 a′ has a first linear polarization 252 that is aligned with the PM fiber of the first receiving waveguide 225 a′. Also, in another implementation, the second component 239 of the return beam 291′ received in the second receiving waveguide 225 b′ has a second linear polarization 254 that is aligned with the PM fiber of the second receiving waveguide 225 b′.

In an implementation, unlike the system 200′″ of FIG. 2I which featured a single free space optic 260 (e.g., a birefringent displacer), the system 200″″ of FIG. 2J features a first free space optic 260 a (e.g., a birefringent displacer) and a second free space optic 260 b (e.g., another birefringent displacer) and one or more polarization transforming optics positioned between the first and second free space optics 260 a, 260 b. In an implementation, the first and second free space optics 260 a, 260 b displace beams in a similar manner as the free space optic 260 of FIG. 2I, including displacing a beam (or component of a beam) with a first linear polarization 252 and not displacing a beam (or component of a beam) with a second linear polarization 254 that is orthogonal to the first linear polarization 252. Additionally, the first and second free space optics 260 a, 260 b may be made from similar materials that are used to make the free space optic 260. Although two birefringent displacers are depicted, in other embodiments more than two birefringent displacers are provided.

In an implementation, as shown in FIG. 2J the transmitted beam 205′ that is non-polarized 253 is incident on the first free space optic 260 a (e.g., a birefringent displacer). In an implementation the first free space optic 260 a displaces a first component 235 of the transmitted beam 205′ by a distance 264 a orthogonal to the direction of travel of the transmitted beam 205′ based on the first linear polarization 252 of the first component 235. In an implementation the first free space optic 260 a does not displace the second component 233 of the transmitted beam 205′ based on the second linear polarization 254 of the second component 233. Thus, in one implementation, the first free space optic 260 a bifurcates the non-polarized transmitted beam 205′ into respective components 233, 235 with orthogonal linear polarization 252, 254. In an implementation, the length 262 a and/or material of the first free space optic 260 a is selected so that the distance 264 a is sized to be about equal to (e.g. within ±10% of) the first separation 221 a. This advantageously shifts the first component 235 of the transmitted beam 205′ to be aligned with the first receiving waveguide 225 a′.

In an implementation, as shown in FIG. 2J one or more polarization transforming optics are provided to adjust a relative phase between orthogonal field components of the transmitted beam 205′ and/or the return beam 291′. In an implementation, the polarization transforming optics include a faraday rotator 266 and a half wave plate 268. In an example implementation, the faraday rotator 266 is made with bismuth iron garnet (BIG) material and/or achieves a faraday rotation of about 45 degrees. In another example implementation, the half wave plate 268 is made of quartz and/or is oriented such that it rotates the polarizations by about 45 degrees. In other implementations, other polarization transforming optics are provided and/or polarization transforming optics that rotate the polarization by angles other than 45 degrees.

In an implementation, the first linear polarization 252 of the first component 235 is rotated by the faraday rotator 266 to an intermediate linear polarization 255 and the second linear polarization 254 of the second component 233 is rotated by the faraday rotator 266 to an intermediate linear polarization 257. Additionally, in an implementation, the intermediate linear polarization 255 of the first component 235 is rotated by the half wave plate 268 to the second linear polarization 254 and the intermediate linear polarization 257 is rotated by the half wave plate 268 to the first linear polarization 252. Thus, in one implementation, the faraday rotator 266 and half wave plate 268 collectively rotate the first linear polarization 252 of the first component 235 to the second linear polarization 254 and further collectively rotate the second linear polarization 254 of the second component 233 to the first linear polarization 252.

In an implementation, the second free space optic 260 b (e.g., a birefringent displacer) displaces the second component 233 of the transmitted beam 205′ incident from the polarization transforming optics based on the first linear polarization 252 of the second component 233. In an implementation, the length 262 b and/or material of the second free space optic 260 b is selected so that the second component 233 is displaced by a distance 264 a that is about equal to (e.g. within ±10% of) the first separation 221 a. In another implementation, the second free space optic 260 b does not displace the first component 235 of the transmitted beam 205′ based on the second linear polarization 254 of the first component 235. In an implementation, the first component 235 and second component 233 of the transmitted beam 205′ recombine based on the displacement of the free space optic 260 b and the transmitted beam 205′ that is non-polarized 253. In one implementation, the reformed transmitted beam 205′ incident on the collimating optics 229 and/or scanning optics 218 is the same as the transmitted beam 205′ incident on the first free space optic 260 a with the exception that the reformed transmitted beam 205′ is laterally shifted to be aligned with the first receiving waveguide 225 a′.

In another implementation, the reformed transmitted beam 205′ is collimated by the collimating optics 229 and/or scanned by the scanning optics 218. In some implementations, the scanning optics 218 is omitted and the reformed transmitted beam 205′ is collimated by the collimating optics 229 at the target 292. The return beam 291′ is reflected from the target 292 and is similarly non-polarized 253. As depicted in FIG. 2J, in one implementation, the nonpolarized return beam 291′ is incident on the second free space optic 260 b (e.g., a birefringent displacer). In an implementation, the second free space optic 260 b displaces a first component 237 of the return beam 291′ based on a first linear polarization 252 of the first component 237. In one implementation, the first component 237 is displaced by the distance 264 a that is about equal to the first separation 221 a. In another implementation, the second free space optic 260 b does not displace a second component 239 of the return beam 291′ based on a second linear polarization 254 of the second component 239 orthogonal to the first linear polarization 252. [0143] In another implementation, the first component 237 and second component 239 of the return beam 291′ are incident on the polarization transforming optics. In an implementation, the polarization transforming optics do not affect the first linear polarization 252 of the first component 237 and the second linear polarization 254 of the second component 239. In one implementation, where the polarization transforming optics include the faraday rotator 266 and the half wave plate 268, the half wave plate 268 rotates the first linear polarization 252 of the first component 237 to an intermediate linear polarization 257 and rotates the second linear polarization 254 of the second component 239 to an intermediate linear polarization 255. In a further implementation, the faraday rotator 266 then rotates the intermediate linear polarization 257 of the first component 237 back to the first linear polarization 252 and rotates the intermediate linear polarization 255 of the second component 239 back to the second linear polarization 254. Thus, in an example implementation the half wave plate 268 and faraday rotator 266 offset one another in terms of the rotation of the linear polarization of the first and second components 237, 239 so that the first and second component 237, 239 are incident on the first free space optic 260 a (e.g., a birefringent displacer) with the same respective linear polarization as incident on the polarization transforming optics. In an example implementation, the half wave plate 268 and faraday rotator 266 rotate the respective linear polarizations by about 45 degrees in equal and opposite directions.

In an implementation, the first component 237 with the first linear polarization 252 and the second component 239 of the return beam 291′ with the second linear polarization 254 are incident on the first free space optic 260 a (e.g., a birefringent displacer). In an implementation, the first free space optic 260 a displaces the first component 237 of the return beam 291′ based on the first linear polarization 252 of the first component 237 and further does not displace the second component 239 of the return beam 291′ based on the second linear polarization 254 of the second component. In an implementation, first component 237 is displaced by a distance 264 b that is based on the second separation 221 b so that the displaced first component 237 is aligned with the second receiving waveguide 225 b′. In an example implementation, one or more parameters of the first free space optic 260 a are selected such as the length 262 a and/or the material of the free space optic 260 a so that the distance 264 b is adjusted to be about equal to the second separation 221 b.

FIG. 2L is a block diagram that illustrates a front view of the bistatic transceiver 215′ of the LIDAR system 200′″ of FIG. 2I, according to an implementation. The left side of FIG. 2L depicts an implementation where no scanning optics 218 and/or no scanning of the transmitted beam 205′ occurs. In an implementation, a displacement vector 274 indicates a direction that the return beam 291′ is displaced by the free space optic 260 (e.g., a birefringent displacer) over the distance 264 so that the return beam 291′ is received at the tip of the receiving waveguide 225′. Since there is no scanning of the transmitted beam 205′, no walk off occurs due to round trip time delay and thus the free space optic 260 is positioned so that the displacement vector 274 is aligned with a vector 275 from the transmission waveguide 223′ to the receiving waveguide 225′.

The middle of FIG. 2L depicts an implementation where scanning optics 218 are employed and scanning of the transmitted beam 205′ occurs in a scan direction 276. In this implementation, the scan direction 276 is angled (e.g. orthogonal) to a vector 275 between the transmission and receiving waveguides 223′, 225′ of the bistatic transceiver 215′. In an implementation, the return beam 291′ is displaced by the free space optic 260 (e.g., a birefringent displacer) along the same displacement vector 274 as in the left side of FIG. 2L that is aligned with the vector 275 between the transmission and receive waveguides 223′, 225′. However, as depicted in the middle of FIG. 2L, a round trip time delay attributable to the scanning of the beam causes beam walk off 277 and hence causes the return beam 291′ to miss the receiving waveguide 225′.

The right of FIG. 2L depicts an implementation where scanning optics 218 are employed and scanning of the transmitted beam 205′ occurs in a scan direction 276. In an implementation, the return beam 291′ is displaced by the free space optic 260 (e.g., a birefringent displacer) along a corrected displacement vector 274′ that is oriented at an angle 278 with respect to the vector 275 between the transmission and receive waveguides 223′, 225′. In one implementation, this is achieved by tilting or orienting the free space optic 260 by the angle 278 relative to the bistatic transceiver 215′ (e.g. in a direction out of the plane of FIG. 2I). In an implementation, the angle 278 is oriented in a same direction as the scan direction 276, relative to the vector 275. In another implementation, a magnitude of the angle 278 is based on a magnitude of the scan speed. In an example implementation, the angle 278 is about 5 degrees or in a range from about 1 degree to about 10 degrees and/or in a range from about 0 degrees to about 20 degrees. In an implementation, the angle 278 is related to the scan speed (e.g. rotation rate of polygon scanner) by:

$\begin{matrix} {\theta = {\arctan\left( \frac{4*{focal}\mspace{14mu}{length}*{rotation}\mspace{14mu}{rate}*{range}}{{waveguide}\mspace{14mu}{separation}\;*c} \right)}} & (8) \end{matrix}$ where θ is the angle 278, focal length is the focal length of the collimating optic 229 (in units of meters); rotation rate is the rotation speed of the polygon scanner 244 (in units of radians per second); range is the target range (in units of meters); waveguide separation is the separation 221 (in units of meters) and c is the speed of light (in units of meters per second). In other implementations, rather than rotating the free space optic 260 (e.g., a birefringent displacer) by the angle 278 (e.g. out of the plane of FIG. 2I) the optical axis of the free space optic 260 is cut by the same angle 278. As depicted in the right side of FIG. 2L, based on the angled displacement vector 274′ and the beam walk off 277 associated with the round trip delay from the scanning, the return beam 291′ is received in the tip of the receiving waveguide 225′.

FIG. 2M is a block diagram that illustrates a front view of the bistatic transceiver 215′ of the LIDAR system 200′″ of FIG. 2I, according to an implementation. The top of FIG. 2M depicts an implementation where no scanning optics 218 and thus no scanning of the transmitted beam 205′ occurs. In an implementation, a displacement vector 274 has a direction based on a direction that the return beam 291′ is displaced by the free space optic 260 (e.g., a birefringent displacer) and a magnitude based on the distance 264 so that the return beam 291′ is received at the tip of the receiving waveguide 225′.

The middle of FIG. 2M depicts an implementation where scanning optics 218 are employed and scanning of the transmitted beam 205′ occurs in a scan direction 276′. In this implementation, the scan direction 276′ is about parallel (e.g. within ±10 degrees) to a vector 275 between the transmission and receiving waveguides 223′, 225′ of the bistatic transceiver 215′. In an implementation, the return beam 291′ is displaced by the free space optic 260 (e.g., a birefringent displacer) along the same displacement vector 274 magnitude as in the left side of FIG. 2L. However, as depicted in the middle of FIG. 2M, beam walk off 277′ attributable to a round trip time delay from scanning of the beam causes the return beam 291′ to miss the receiving waveguide 225′ between the transmission waveguide 223′ and receiving waveguide 225′ and thus the displacement vector 274 magnitude (e.g. the distance 264) was too small.

The bottom of FIG. 2M depicts an implementation where scanning optics 218 are employed and scanning of the transmitted beam 205′ occurs in a scan direction 276′. In an implementation, the return beam 291′ is displaced by the free space optic 260 (e.g., a birefringent displacer) along a corrected displacement vector 274″ that has an increased magnitude (e.g. increased distance 264) so that the beam walk off 277′ due to the round trip delays results in the return beam 291′ being received in the receiving waveguide 225′. In some implementations, the displacement vector magnitude is adjusted by adjusting the length 262 of the free space optic 260. In an example implementation, the distance 264 that the return beam 291′ is displaced is increased by increasing the length 262 of the free space optic 260. In an implementation, a variation of the length 262 is a ratio (e.g. about 10×) of the variation of the displacement distance 264. In an example implementation, the length 262 is varied by less than about 200 μm or in a range from about 100 μm to about 300 μm. In another example implementation, the variation of the length 262 is positive or negative depending on a scan direction relative to the bistatic transceiver.

FIG. 2N is a block diagram that illustrates a top view of example components of a bistatic LIDAR system 200′″, according to an implementation. In an implementation, FIG. 2N depicts the free space optic 260 (e.g., a birefringent displacer) is oriented at the angle 278 relative to the direction or axis 275 between the transmission and receiving waveguides 223′, 225′ within the bistatic transceiver 215′. Thus, in an implementation, the return beam 291′ is displaced by the free space optic 260 by the distance 264 at the angle 278 relative to the bistatic transceiver 215′ so that the return beam 291′ is received at the tip of the receiving waveguide 225′ when beam walk off 277 associated with round trip time delays are considered. Additionally, in an implementation, FIG. 2N depicts that the one or more transmission waveguides 223′ of the bistatic transceiver 215′ transmits a fan of diverging transmitted beams 205 a′, 205 b′ which are subsequently collimated into a fan of collimated transmitted beams 205 a″, 205 b″ by the collimating optics 229 and/or scanning optics 218. In an example implementation, the diverging transmitted beams 205 a′, 205 b′ expand to a diameter of about 8 millimeters (mm) or in a range from about 4 mm to about 12 mm and/or in a range from about 2 mm to about 20 mm at the collimating optics 229. However, in other implementations a diameter of the diverging transmitted beams 205 a′, 205 b′ is varied to meet the needs of physical size and range performance of the LIDAR system. In another example implementation, the collimating optics 229 is an aspheric lens with a focal length in a range from about 40 mm to about 50 mm and/or in a range from about 30 mm to about 60 mm which collimates the transmitted beams 205 a″, 205 b″. Similarly, the collimating optics 229 receives a fan of collimated return beams (not shown) which are largely overlapped with their respective collimated transmitted beams 205 a″, 205 b″. In an example implementation, an angular separation between the collimated transmitted beams 205 a″, 205 b″ and the fan of collimated return beams is on the order of about 0.02 degrees to about 0.05 degrees and/or on the order of about 0.01 degrees to about 0.10 degrees. In an implementation, the collimating optics 229 focuses the fan of collimated return beams into converging return beams at the quarter wave plate 211 of FIG. 2N. In an implementation, the fine dotted lines below the dashed lines between the fiber array 215′ and the free space optic 260 in FIG. 2N (e.g., a birefringent displacer) represent the separated return beams entering the adjacent receiving waveguides 225′. In an implementation, the transceiver of FIG. 2N advantageously provides an arrangement where two transmit waveguides create a fan of transmitted beams from a single collimator.

FIG. 2O is a block diagram that illustrates a top view of example components of a bistatic LIDAR system 200″″, according to an implementation. In an implementation, the bistatic transceiver 215″ of FIG. 2O includes one or more groups of waveguides, where each group of waveguides is the transmission waveguide 223 and the pair of receiving waveguides 225 a′, 225 b′ on opposite sides of the transmission waveguide 223. In an example implementation, the transmission waveguide 223 is a SM fiber and the pair of receiving waveguides 225 a′, 225 b′ are PM fibers with orthogonal orientation. In an example implementation, the bistatic transceiver 215″ includes multiples of three waveguides, where each group of three waveguides is the transmission waveguide 223 and pair of receiving waveguides 225 a′, 225 b′ on opposite sides of the transmission waveguide 223. Additionally, in an implementation, FIG. 2O depicts that the one or more transmission waveguide 223 of the bistatic transceiver 215″ transmits a pair of diverging transmitted beams which are subsequently collimated into a fan of collimated transmitted beams (e.g. as depicted in FIG. 2N) by the collimating optics 229 and/or scanning optics 218. In an example implementation, the diverging transmitted beams expand to a diameter of about 8 millimeters (mm) or in a range from about 4 mm to about 12 mm and/or in a range from about 2 mm to about 20 mm at the collimating optics 229. In another example implementation, the collimating optics 229 is an aspheric lens with a focal length in a range from about 40 mm to about 50 mm and/or in a range from about 30 mm to about 60 mm which collimates the transmitted beam 205″. Similarly, the collimating optics 229 receives a fan of collimated return beams 291 a″, 291 b″ and focuses converging return beams 291 a′, 291 b′ to the free space optic 260 b of FIG. 2J. As with the implementation of FIG. 2N, the fan of collimated return beams 291 a″, 291 b″ are largely overlapped with the fan of collimated transmitted beams. In an example implementation, the angular separation between the fan of collimated return beams 291 a″, 291 b″ and the collimated transmitted beams is on the order of about 0.02 to about 0.05 degrees and/or on the order of about 0.01 degrees to about 0.10 degrees. In an implementation, the transceiver of FIG. 2O advantageously provides an arrangement where two transmit waveguides create a fan of transmitted beams from a single collimator.

FIG. 6B is a flow chart that illustrates an example method 650 for spatial displacement of transmit and receive modes in a bistatic LIDAR system, according to an implementation. In an implementation, in step 651 the beam 205′ is transmitted from the laser source 212 and through the transmission waveguide 223′, 223 of the bistatic transceiver. In some implementations, in step 651 where the beam 205′ is nonpolarized the beam 205′ is transmitted from the transmission waveguide 223 (e.g. SM fiber) of the bistatic transceiver 215″ of FIG. 2J. In other implementations, in step 651 where the beam 205′ is linearly polarized the beam 205′ is transmitted from the transmission waveguide (e.g. PM fiber) of the bistatic transceiver 215′ of FIG. 2I.

In an implementation, in step 653 the transmitted beam 205′ is displaced with a birefringent displacer in a direction orthogonal to the direction of the transmitted beam 205′. In one implementation, in step 653 a first component 235 of the transmitted beam 205′ in the system 200″″ of FIG. 2J is displaced by the distance 264 a orthogonal to the direction of the transmitted beam 205′. In an example implementation, the distance 264 a is based on the first separation 221 a so that the displaced first component 235 is aligned with the first receiving waveguide 225 a′. In some implementations, such as the system 200′″ of FIG. 2I, step 653 is omitted.

In an implementation, in step 655 the relative phase between orthogonal field components of the transmitted beam 205′ is adjusted with a polarization transforming optic. In one implementation, in step 655 the first linear polarization 252 of the transmitted beam 205′ is adjusted to a first circular polarization 256 using a quarter wave plate 211 (FIG. 2I). In another implementation, in step 655 the first linear polarization 252 of the first component 235 of the transmitted beam 205′ is rotated to a second linear polarization 254 orthogonal to the first linear polarization 252 using a faraday rotator 266 and half wave plate 268 (FIG. 2J). Similarly, in step 655 the second linear polarization 254 of the second component 233 of the transmitted beam 205′ is rotated to the first linear polarization 252 using the faraday rotator 266 and half wave plate 268 (FIG. 2J).

In an implementation, in step 655 the relative phase between orthogonal field components of the return beam 291′ is adjusted with a polarization transforming optic. In one implementation, in step 655 the second circular polarization 258 of the return beam 291′ is adjusted to the second circular polarization 254 using a quarter wave plate 211 (FIG. 2I).

In an implementation, in step 657 the collimating optic 229 is used to shape the transmitted beam 205′ from the transmission waveguide and to shape the return beam 291′ reflected from the target 292. In one implementation, in step 657, the collimating optic 229 has a focal length such that a diverging transmitted beam 205′ (e.g. diameter of about 8 mm at the collimating optic 229) is collimated by the optic 229. In an example implementation, the collimating optic 229 has a focal length that is sized (e.g. in a range from about 40 mm to about 50 mm) so that the diverging transmitted beam 205′ is collimated into a collimated transmitted beam 205″.

In an implementation, in step 658 the return beam 291′ is displaced with a birefringent displacer in a direction orthogonal to the direction of the return beam 291′. In one implementation, in step 658 the return beam 291′ in the system 200′″ of FIG. 2I is displaced by the distance 264 orthogonal to the direction of the return beam 291′. In an example implementation, the distance 264 is based on the separation 221. In another implementation, in step 658 a first component 237 of the return beam 291′ in the system 200″″ of FIG. 2J is displaced by the distance 264 a orthogonal to the direction of the return beam 291′. In an example implementation, the distance 264 a is based on the first separation 221 a so that the displaced first component 235 is aligned with the first receiving waveguide 225 a′.

In an implementation, in step 659 the return beam 291′ is received at one or more receiving waveguides of the bistatic transceiver. In one implementation, the return beam 291′ is received at the receiving waveguide 225′ of the system 200′″ of FIG. 2I. In another implementation, first and second components 237, 239 of orthogonal linear polarization of the return beam 291′ are received in respective receiving waveguides 225 a′. 225 b′.

In an implementation, in step 661 the received return beam 291′ in step 659 is combined with a reference signal 207 (or LO signal) in one or more optical mixers 284. In some implementations, where multiple components 237, 239 of orthogonal linear polarization of the return beam 291′ are received in respective receiving waveguides 225 a′, 225 b′, multiple processing channels are provided for each respective component so separately process each respective component of the return beam 291′. In an implementation, in step 663 a signal is generated based on the combining in step 661 and in step 665 a device is operated based on the generated signal. In some implementations, the device is a vehicle such as a braking or acceleration or steering system of the vehicle.

6. Vehicle Control Overview

In some implementations a vehicle is controlled at least in part based on data received from a hi-res Doppler LIDAR system mounted on the vehicle.

FIG. 3A is a block diagram that illustrates an example system 301 that includes at least one hi-res Doppler LIDAR system 320 mounted on a vehicle 310, according to an implementation. In an implementation, the LIDAR system 320 is similar to one of the LIDAR systems 200, 200′, 200″, 200′″, 200″″. The vehicle has a center of mass indicted by a star 311 and travels in a forward direction given by arrow 313. In some implementations, the vehicle 310 includes a component, such as a steering or braking system (not shown), operated in response to a signal from a processor, such as the vehicle control module 272 of the processing system 250. In some implementations the vehicle has an on-board processor 314, such as chip set depicted in FIG. 8. In some implementations, the on board processor 314 is in wired or wireless communication with a remote processor, as depicted in FIG. 7. In an implementation, the processing system 250 of the LIDAR system is communicatively coupled with the on-board processor 314 or the processing system 250 of the LIDAR is used to perform the operations of the on board processor 314 so that the vehicle control module 272 causes the processing system 250 to transmit one or more signals to the steering or braking system of the vehicle to control the direction and speed of the vehicle. The hi-res Doppler LIDAR uses a scanning beam 322 that sweeps from one side to another side, represented by future beam 323, through an azimuthal field of view 324, as well as through vertical angles (FIG. 3B) illuminating spots in the surroundings of vehicle 310. In some implementations, the field of view is 360 degrees of azimuth. In some implementations the inclination angle field of view is from about +10 degrees to about −10 degrees or a subset thereof. In some implementations, where the system 320 is the system 200″, the field of view 324 is defined by the range of angles 227. In designing the system 301, a predetermined maximum design range of the beams at each angle over the field of view 324 is determined and represents a maximum anticipated target range at each angle in the range field of view 324. In an example implementation, the maximum design range is a fixed value or a fixed range of values over the field of view 324. In an example implementation, the maximum design range over the field of view 324 is about 200 meters or in a range from about 150 meters to about 300 meters.

In some implementations, the vehicle includes ancillary sensors (not shown), such as a GPS sensor, odometer, tachometer, temperature sensor, vacuum sensor, electrical voltage or current sensors, among others well known in the art. In some implementations, a gyroscope 330 is included to provide rotation information.

FIG. 3B is a block diagram that illustrates an example system 301′ that includes at least one hi-res LIDAR system 320 mounted on the vehicle 310, according to an implementation. In an implementation, the LIDAR system 320 is similar to the system 200 or system 200′ or system 200″. In one implementation, the vehicle 310 moves over the surface 349 (e.g. road) with the forward direction based on the arrow 313. The LIDAR system 320 scans over a range of angles 326 from a first beam 342 oriented at a first angle measured with respect to the arrow 313 to a second beam 346 oriented at a second angle measured with respect to the arrow 313. In one implementation, the first angle and the second angle are vertical angles within a vertical plane that is oriented about orthogonal with respect to the surface 349. For purposes of this description, “about orthogonal” means within ±20 degrees of a normal to the surface 349. In some implementations, where the LIDAR system 320 is similar to the system 200″, the range of angles 326 is defined by the range of angles 227.

In designing the system 301′, a predetermined maximum design range of the beams at each angle is determined and represents a maximum anticipated target range at each angle in the range 326. In other implementations, the maximum design range at each angle is not predetermined but is regularly measured and updated in the memory of the processing system 250 at incremental time periods. In an implementation, the first beam 342 is oriented toward the surface 349 and intersects the surface 349 within some maximum design range from the vehicle 310. Thus, at the first angle the system 320 does not consider targets positioned beyond the surface 349. In an example implementation, the first angle of the first beam 342 is about −15 degrees or in a range from about −25 degrees to about −10 degrees with respect to the arrow 313 and the maximum design range is about 4 meters (m) or within a range from about 1 m to about 10 m or in a range from about 2 m to about 6 m. In an implementation, the second beam 346 is oriented toward the sky and intersects a ceiling 347 within some maximum design range from the vehicle 310. Thus, at the second angle the system 320 does not consider targets positioned above the ceiling 347. In an example implementation, the ceiling 347 is at an altitude of about 12 m or in a range from about 8 m to about 15 m from the surface 349 (e.g. that defines an altitude of 0 m), the second angle of the second beam 346 is about 15 degrees or in a range from about 10 degrees to about 20 degrees with respect to the arrow 313 and the maximum design range is about 7 m or within a range from about 4 m to about 10 m or within a range from about 1 m to about 15 m. In some implementations, the ceiling 347 altitude depends on an altitude of the LIDAR system 320 (e.g. about 1.5 m or in a range of about 1 m to about 4 m, where the surface 349 defines 0 m). In an implementation, an intermediate beam 344 between the first beam 342 and second beam 346 is oriented about parallel with the arrow 313 and intersects a target 343 positioned at a maximum design range from the vehicle 310. In one example implementation, FIG. 3B is not drawn to scale and target 343 is positioned at a much further distance from the vehicle 310 than depicted. For purposes of this description, “about parallel” means within about ±10 degrees or within about ±15 degrees of the arrow 313. In an example implementation, the maximum design range of the target 343 is about 200 m or within a range from about 150 m to about 300 m or within a range from about 100 m to about 500 m.

Although FIG. 3B depicts the LIDAR system mounted on a vehicle 310 configured to travel over the surface 349, the implementations of the present is not limited to this type of vehicle and the LIDAR system can be mounted to an air vehicle 310′ (e.g. passenger air vehicle) that is configured to fly. In an implementation, the vehicle 310′ is configured to fly over a surface 349 where one or more targets 343 are present. FIG. 3D is a block diagram that illustrates an example system 301″ that includes at least one hi-res LIDAR system 320 mounted on a vehicle 310′ configured to fly over a surface 349, according to an implementation. In an implementation, the LIDAR system 320 operates in a similar manner as the LIDAR system 320 of the system 301′, with the exception that the maximum design range of the first beam 342′ at the first angle with respect to the arrow 313 is defined based on a floor 348 relative to the surface 349. In an example implementation, the floor 348 has an altitude relative to the altitude of the system 320 in a range from about 0 m to about −10 m or in a range from about 0 m to about −2 m. In another example implementation, the ceiling 347 has an altitude relative to the altitude of the system 320 in a range from about 0 m to about 10 m. In another example implementation, the first angle is about −30 degrees or within a range from about −60 degrees to about −15 degrees. In some implementations, the first angle would be equal and opposite to the second angle of the ceiling 347.

7. Method for Optimization of Scan Pattern in Coherent LIDAR System

FIG. 6A is a flow chart that illustrates an example method 600 for optimizing a scan pattern of a LIDAR system. In some implementations, the system 600 is for optimizing the scan pattern of a LIDAR system mounted on an autonomous vehicle. Although steps are depicted in FIG. 6A and in FIG. 6B, and in subsequent flowcharts as integral steps in a particular order for purposes of illustration, in other implementations, one or more steps, or portions thereof, are performed in a different order, or overlapping in time, in series or in parallel, or are omitted, or one or more additional steps are added, or the method is changed in some combination of ways.

In step 601, data is received on a processor that indicates first SNR values of a signal reflected by a target and received by a receiving waveguide of a bistatic transceiver after the signal is transmitted by a transmission waveguide of the bistatic transceiver, where the receiving waveguide is spaced apart from the transmission waveguide by a separation. The first SNR values are based on values of a range of the target and the first SNR values are for a respective value of a scan rate of the LIDAR system. In an implementation, in step 601 the first SNR values are of the return beam 291′ reflected by the target and received by the receiving waveguide 225 of the bistatic transceiver 215 after the beam 205′ is transmitted by the transmission waveguide 223, where the receiving waveguide 225 is spaced apart from the transmission waveguide 223 by the separation 221. In one implementation, the data is first SNR values of the focused return beam 291′ on the tip 217 of either or both of the receiving waveguides 225 a, 225 b in the system 200″. In another implementation, in step 601 the data is first SNR values of the focused return beam 291′ on the detector array 230 in the system 200″. In one implementation, the data includes values of curve 440 b and/or curve 440 c and/or curve 440 d that indicate SNR values of the return beam 291′, where each curve 440 is for a respective value of the scan rate of the beam. In an example implementation, the curves 440 b, 440 c, 440 d are based on the same value of the separation 221 and in step 601 a plurality of sets of curves 440 b, 440 c, 440 d are received for a respective plurality of values of the separation 221. In some implementations, the data is not limited to curves 440 b, 440 c, 440 d and includes SNR values of less or more curves than are depicted in FIG. 4G, where each SNR curve is based on a respective value of the scan rate and/or the curves 440 b, 440 c, 440 d are based on a particular value of the separation 221.

In other implementations, the data received in step 601 includes SNR values that could be used to form the curve over the target range for each respective value of the scan rate and/or each value of the separation 221. In one implementation, curves 464, 466 are provided based on a fixed value of the scan rate (e.g. 4000 deg/sec) and for multiple values of the separation 221. In another implementation, curves 465, 467 are provided based on a fixed value of the scan rate (e.g. 12000 deg/sec) and for multiple values of the separation 221. In yet another implementation, in step 601 curves 469 are provided for a respective target range in order to achieve an SNR threshold and the curves 469 indicate a required separation 221 value for a particular fixed scan speed. In an example implementation, in step 601 the data is stored in a memory of the processing system 250 and each set of first SNR values is stored with an associated value of the scan rate of the LIDAR system and/or an associated value of the separation 221. In one implementation, in step 601 the first SNR values are obtained over a range from about 0 meters to about 500 meters (e.g. automotive vehicles) or within a range from about 0 meters to about 1000 meters (e.g. airborne vehicles) and for scan rate values from about 2000 deg/sec to about 6000 deg/sec or within a range from about 1000 deg/second to about 7000 deg/sec and/or for separation 221 values in a range from about 0w_(o) to 4 w_(o) or from about 0w_(o) to about 10w_(o) where w_(o) is the diameter of the transmission waveguide 223. In some implementations, the first SNR values are predetermined and are received by the processor in step 601. In other implementations, the first SNR values are measured by the LIDAR system and subsequently received by the processor in step 601. In one implementation, the data is input in step 601 using an input device 712 and/or uploaded to the memory 704 of the processing system 250 over a network link 778 from a local area network 780, internet 790 or external server 792.

In an implementation, in step 601 the first SNR values (e.g. curves 440 b, 440 c, 440 d) are received for the return beam 291′ received by the first receiving waveguide 225 a based on the first separation 221 a and another set of first SNR values (e.g. curves 440 b, 440 c, 440 d) are received for the return beam 291′ received by the second receiving waveguide 225 b based on the second separation 221 b. In one example implementation, the first SNR values for the return beam 291′ received by the receiving waveguides 225 a, 225 b are predetermined and received by the processor in step 601. In another example implementation, the first SNR values for the return beam 291′ received by the receiving waveguides 225 a, 225 b are measured by the LIDAR system and subsequently received by the processor in step 601.

In step 603, data is received on a processor that indicates second SNR values of a signal reflected by a target and detected by the LIDAR system based on values of a range of the target, where the second SNR values are for a respective value of an integration time of the LIDAR system. In an implementation, in step 603 the data is second SNR values of the focused return beam 291′ in the system 200″ for a respective integration time over which the beam is processed by the acquisition system 240 and/or processing system 250. In one implementation, the data includes values of curve 450 a and/or curve 450 b and/or curve 450 c and/or curve 450 d that indicate SNR values of the return beam 291′, where each curve 450 is for a respective value of the integration time that the beam is processed by the acquisition system 240 and/or processing system 250. In some implementations, the data is not limited to curves 450 a, 450 b, 450 c, 450 d and includes less or more curves than are depicted in FIG. 4I, where each SNR curve is based on a respective value of the integration time. In other implementations, the data need not be a curve and instead is the SNR values used to form the curve over the target range for each respective value of the integration time. In an example implementation, in step 603 the data is stored in a memory of the processing system 250 and each set of second SNR values is stored with an associated value of the integration time of the LIDAR system. In one implementation, in step 603 the second SNR values are obtained over a range from about 0 meters to about 500 meters (e.g. automotive vehicles) or from a range from about 0 meters to about 1000 meters (e.g. airborne vehicles) and for integration time values from about 100 nanosecond (ns) to about 5 microseconds (μs). In some implementations, the second SNR values are predetermined and are received by the processor in step 603. In other implementations, the second SNR values are measured by the LIDAR system and subsequently received by the processor in step 603. In one implementation, the data is input in step 603 using an input device 712 and/or uploaded to the memory 704 of the processing system 250 over a network link 778 from a local area network 780, internet 790 or external server 792.

In step 605, data is received on a processor that indicates the first angle and the second angle that defines the angle range 227. In one implementation, in step 605 the first angle and the second angle are input using an input device 712 (e.g. mouse or pointing device 716) and/or are uploaded to the processing system 250 over a network link 778. In one implementation, where the angle range 227 is the field of view 324 of FIG. 3A, the first angle is defined as the angle between the first beam 322 and the arrow 313 indicating the direction of travel of the vehicle 310 and the second angle is defined as the angle between the second beam 323 and the arrow 313.

In another implementation, where the angle range 227 is the angle range 326 of FIG. 3B, the first angle is defined as the angle between the first beam 342 and the arrow 313 indicating the direction of travel of the vehicle 310 and the second angle is defined as the angle between the second beam 346 and the arrow 313. In an implementation, the first angle and second angle are symmetric with respect to the arrow 313, e.g. the first angle and the second angle are equal and opposite to each other. In one implementation, the first angle is selected so that the first beam 342 is oriented towards the surface 349 and the second angle is selected so that the second beam 346 is oriented away from the surface 349 and towards the ceiling 347.

In one implementation, steps 601, 603 and 605 are simultaneously performed in one step where the data in steps 601, 603 and 605 is received at the processor in one simultaneously step.

In step 607, data is received on a processor that indicates the maximum design range of the target at each angle in the angle range. In an implementation, the maximum design range is the predetermined maximum range of the target at each angle in the range of angles 227. In one implementation, in step 607 the maximum design range of the target at each angle is based on the field of view 324 of FIG. 3A. In an example implementation, the maximum design range is a fixed value or a fixed range of values over the field of view 324. In an example implementation, the maximum design range over the field of view 324 is about 250 meters or in a range from about 150 meters to about 300 meters.

In another implementation, the data in step 607 is provided over a first range of angles that is greater than the range of angles 326. In one implementation, the data in step 607 is provided at an incremental angle over the angle range, wherein the incremental angle is selected in a range from about 0.005 degrees to about 0.01 degrees or in a range from about 0.0005 degrees to about 0.01 degrees.

In one example implementation, the data in step 607 is input using an input device 712 (e.g. mouse or pointing device 716) and/or are uploaded to the processing system 250 over a network link 778. In some implementations, the maximum design range is predetermined and received during step 607. In other implementations, the system 200, 200′, 200″ is used to measure the maximum design range at each angle in the angle range 227 and the maximum design range at each angle is subsequently received by the processing system 250 in step 607.

In step 609, a maximum scan rate of the LIDAR system is determined at each angle in the angle range 227 so that the SNR of the LIDAR system is greater than a minimum SNR threshold. In an implementation, in step 609 a fixed maximum scan rate is determined for the angles in the angle range 227. A range of values for the maximum design range over the angle range 227 is first determined based on the received data in step 607. First SNR values received in step 601 are then determined for the value or range of values of the maximum design range (e.g. about 150 m to about 300 m) over the angle range 227 and it is further determined which of these first SNR values exceed the minimum SNR threshold. In one implementation, values of curves 440 b, 440 c, 440 d are determined for the range of values of the maximum design range (e.g. about 80 m-about 120 m) over the angle range 227 and it is further determined that only the values of curve 440 d exceeds the minimum SNR threshold 442 for the range of values of the maximum design range (e.g. about 80 m-about 120 m). Since only the values of curve 440 d exceeds the minimum SNR threshold 442, the fixed maximum scan rate over the angle range 227 is set to the scan rate corresponding to the curve 440 d. In an example implementation, the determining of the maximum scan rate in step 609 ensures that beam walkoff 419, 421 (FIG. 4E) of the return beam 291′ on the tip 217 of the receiving waveguides 225 a, 225 b is within a threshold of the separation 221, where the threshold is based on a diameter of the image 418 of the return beam 291′ on the tip (e.g. larger threshold for larger diameter, smaller threshold for smaller diameter). In an example implementation, the threshold is a ratio of a diameter of the image 418 of the return beam 291′ on the tip 217 of the receiving waveguide 225. In an example implementation, the ratio is about 0.5 or in a range from about 0.3 to about 0.7.

In another implementation, in step 609, first SNR values (e.g. curves 464, 465, 466, 467, 469) corresponding to different values of the separation 221 are used to determine first SNR values (e.g. one of the curves 464 for fixed scan speed 4000 deg/sec or one of curves 465 for fixed scan speed 12000 deg/sec) that exceeds the minimum SNR threshold 442 and is the maximum value among those first SNR values which exceed the minimum SNR threshold 442. In an implementation, during a design phase of the system 200″ the separation 221 between the transmission waveguide 223 and the receiving waveguide 225 is selected based on the value of the separation 221 corresponding to the determined first SNR values and the fixed maximum scan rate is selected based on the scan rate corresponding to the determined first SNR values (e.g. the value of the separation 221 is selected at 2.75w_(o) based on curve 465 c for target design range of 0 m-250 m and/or the fixed scan rate is selected based on the fixed scan rate of 12,000 deg/sec for curves 465). In one example implementation, first SNR values (e.g. curves 464 c) corresponding to the first separation 221 a (e.g. 0.25 w_(o)) is used for a fixed scan rate (e.g. 4000 deg/sec) so that the SNR exceeds the SNR threshold 442 over a first portion of the angle range 227 (e.g. for target range >80 m) and first SNR values (e.g. curve 464 a) corresponding to the second separation 221 b (e.g. 4w_(o)) is used for the fixed scan rate so the SNR exceeds the SNR threshold 442 over a second portion of the angle range 227 (e.g. for target range <80 m). In this implementation, the fixed maximum scan rate over the angle range 227 is set to the optimal scan rate. This implementation advantageously uses multiple receiving waveguides 225 a, 225 b to detect return beam 291′ data from different portions of the maximum design range (e.g. receiving waveguide 225 a receives return beams 291′ from targets at longer range, receiving waveguide 225 b receives return beams 291′ from targets at shorter range) over the angle range 227 while the scan rate is fixed over the angle range 227. In an example implementation, where the angle range 227 is the angle range 326, the receiving waveguide 225 b receives return beams 291′ based on the transmitted beams 342, 346 at the first and second angles and the receiving waveguide 225 a receives return beams 291′ based on the intermediate transmitted beam 344.

In another implementation, in step 609 a respective maximum scan rate is determined for each angle in the angle range 227. At each angle, the maximum design range for that angle is first determined based on the received data in step 607. First SNR values received in step 601 are then determined for the maximum design range at the angle and it is further determined which of these first SNR values exceed the minimum SNR threshold. In one implementation, values of curves 440 b, 440 c, 440 d are determined for a maximum design range (e.g. about 90 m) and it is further determined that the values of curves 440 b, 440 c exceeds the minimum SNR threshold 442. Among those first SNR values which exceed the minimum SNR threshold, the first SNR values with the maximum scan rate is selected and the maximum scan rate is determined in step 609 for that angle. In the above implementation, among the values of the curves 440 b, 440 c which exceeds the minimum SNR threshold 442 at the maximum design range (e.g. about 90 m), the curve 440 c values are selected as the maximum scan rate and the maximum scan rate (e.g. optimal scan rate associated with curve 440 c) is determined in step 609 for that angle. In one example implementation, FIG. 4G depicts that the maximum scan rate determined in step 609 for the beams 342, 346 at the first and second angle (e.g. optimal scan rate based on curve 440 c) is greater than the maximum scan determined in step 609 for beam 344 at an angle between the first and second angle (e.g. slow scan rate based on curve 440 b). In this example implementation, the scan rate of the scanning optics 218 is varied over the angle range 227 and the return beam 291′ is scanned at a fast scan rate for that portion of the angle range 227 corresponding to shorter target range (e.g. beams 342, 346) and is scanned at a slow scan rate for that portion of the angle range 227 corresponding to a longer target range (e.g. beam 344). In this example implementation, one receiving waveguide 225 is used to capture returns beams 291′ over the angle range 227. In an example implementation, the determining of the maximum scan rate in step 609 ensures that beam walkoff 419 (FIG. 4E) of the return beam 291′ on the tip 217 of the receiving waveguide 225 is within a threshold of the separation 221, where the threshold is less than a ratio of a diameter of the image 418 of the return beam 291′ on the tip 217. In an example implementation, the ratio is about 0.5 or in a range from about 0.3 to about 0.7.

FIG. 3C is a block diagram that illustrates an example of transmitted beams 343 a, 343 b at multiple angles 345 a, 345 b from the LIDAR system 320 of FIG. 3B, according to an implementation. In one implementation, beams 343 a, 343 b are intermediate beams between first beam 342 and intermediate beam 344. In other implementations, beam 343 a is the first beam 342 and beam 343 b is a subsequent beam that is processed after the first beam 342. In step 609, the maximum scan rate of the LIDAR system 320 is determined at the angle 345 a. The maximum design range (e.g. 30 m) of the beam 343 a at the angle 345 a is first determined using the data in step 607. First SNR values from step 601 for the maximum design range are then determined. In an example implementation, the first SNR values include values of curves 440 b, 440 c, 440 d. It is then determined which of those first SNR values at the maximum design range exceeds the minimum SNR threshold. In the example implementation, the values of the curves 440 b, 440 c, 440 d at the maximum design range (e.g. 30 m) all exceed the minimum SNR threshold 442. It is then determined which of these first SNR values has the maximum scan rate and this maximum scan rate is determined in step 609 for that angle. In the example implementation, the curve 440 c has the maximum scan rate and thus this maximum scan rate is used to scan the beam 343 a at the angle 345 a. In an implementation, FIG. 4I depicts that the minimum scan rate determined in step 611 for the beams 342, 346 at the first and second angle (e.g. 400 ns based on curve 450 d) is shorter than the minimum integration time determined in step 611 for beam 344 at an angle between the first and second angle (e.g. 3.2 μs based on curve 450 a).

In step 611, a minimum integration time of the LIDAR system is determined at each angle in the angle range 227 so that the SNR of the LIDAR system is greater than a minimum SNR threshold. In some implementations, where the maximum design range has a fixed value or a fixed range of values over the angle range 227, in step 611 a fixed minimum integration time is determined over the angle range 227 based on the fixed maximum design range (e.g. 200 m) or fixed range of values of the maximum design range (e.g. 180 m-220 m). In other implementations, at each angle in the angle range 227, the maximum design range for that angle is first determined based on the received data in step 607. Second SNR values received in step 603 are then determined for the maximum design range at the angle and it is further determined which of these second SNR values exceed the minimum SNR threshold. In one implementation, values of curves 450 a, 450 b, 450 c, 450 d are determined for a maximum design range (e.g. about 120 m) or range of values for the maximum design range and it is further determined that the values of curves 450 a, 450 b, 450 c exceeds the minimum SNR threshold 452. Among those second SNR values which exceed the minimum SNR threshold, the second SNR values with the minimum integration time is selected and the minimum integration time is determined in step 611 for that angle or range of angles 227. In the above implementation, among the values of the curves 450 a, 450 b, 450 c which exceeds the minimum SNR threshold 452 at the maximum design range (e.g. about 120 m), the curve 450 c values are selected with the minimum integration time and the minimum integration time (e.g. about 800 ns) is determined in step 611 for that angle.

In step 611, the minimum integration time of the LIDAR system 320 is determined at the angle 345 a. The maximum design range (e.g. 30 m) of the beam 343 a at the angle 345 a is first determined using the data in step 607. Second SNR values from step 603 for the maximum design range are then determined. In an example implementation, the second SNR values include values of curves 450 a, 450 b, 450 c, 450 d. It is then determined which of those second SNR values at the maximum design range exceeds the minimum SNR threshold. In the example implementation, the values of the curves 450 a, 450 b, 450 c, 450 d at the maximum design range (e.g. 30 m) all exceed the minimum SNR threshold 452. It is then determined which of these second SNR values has the minimum integration time and this minimum integration time is determined in step 611 for that angle. In the example implementation, the curve 450 d has the minimum integration time (e.g. about 400 ns) and thus this minimum integration time is used to process the beam 343 a at the angle 345 a.

In step 613, a determination is made whether additional angles remain in the angle range 227 to perform another iteration of steps 609, 611. In some implementations, where the maximum design range has a fixed value or a fixed range of values over the angle range 227, steps 609, 611 are each performed once based on this fixed value or fixed range of values of the maximum design range, steps 613 and 615 are omitted and the method 600 proceeds to step 617. In this implementation, a fixed maximum scan rate is determined in step 609, based on the fixed value or fixed range of values of the maximum design range and a fixed minimum integration time is determined in step 611 based on the fixed value or fixed range of values of the maximum design range.

In one implementation, in step 613 for the angle range 326, where an initial iteration of steps 609, 611 was at the first angle of the first beam 342 in the range 326, step 613 involves a determination of whether the previous iteration of steps 609, 611 was at or beyond the second angle of the second beam 346 in the angle range 326. In another implementation, where an initial iteration of the steps 609, 611 was at the second angle of the range 326, step 613 involves a determination of whether the previous iteration of steps 609, 611 was at or beyond the first angle of the angle range 326. If step 613 determines that more angles in the range 326 remain, the method proceeds to block 615. If step 613 determines that no further angles in the range 326 remain, the method proceeds to block 617.

In step 615, a determination is made of a subsequent angle at which to iterate steps 609, 611 after having performed a previous iteration of steps 609, 611 at an initial angle. In an implementation, FIG. 3C depicts a subsequent angle 345 b of the subsequent beam 343 b at which to iterate steps 609, 611 after having performed the previous iteration of steps 609, 611 at the initial beam 343 a at the initial angle 345 a. In an implementation, step 615 involves a determination of the subsequent angle 345 b and angle increment 350 a between the initial angle 345 a and the subsequent angle 345 b. In one implementation, the subsequent angle 345 b is based on the initial angle 345 a, the maximum scan rate at the angle 345 a determined in step 609 and the minimum integration time at the angle 345 a determined in step 611. In an example implementation, the subsequent angle θ_(s) is based on the initial angle θ_(i), maximum scan rate S_(m) and minimum integration time I_(m) using: θ_(s)=θ_(i) +S _(m) I _(m)  (9) In an example implementation, if the initial angle 345 a is −15 degrees, the maximum scan rate is 15 degrees per second and the minimum integration time is 2 μs, then the subsequent angle 345 b is about −14.97 degrees using equation 9. After determining the subsequent angle in step 615, the method proceeds back to block 609 so that steps 609, 611 are iterated at the subsequent angle.

In step 617, after it is determined that no further iterations of steps 609, 611 need to be performed, a scan pattern of the LIDAR system is defined based on the maximum scan rate from step 609 and the minimum integration time from step 611 at each angle in the angle range 326. In some implementations, where the maximum design range has a fixed value or a fixed range of values over the angle range 227, the fixed maximum scan rate is determined in step 609, based on the fixed value or fixed range of values of the maximum design range and the fixed minimum integration time is determined in step 611 based on the fixed value or fixed range of values of the maximum design range. In this implementation, in step 617 the scan pattern is defined based on the fixed maximum scan rate and the fixed minimum integration time at each angle over the angle range 227.

In another implementation, the scan pattern includes the maximum scan pattern and minimum integration time for each angle in the angle range 326 between the first angle of the first beam 342 and the second angle of the second beam 346. In an example implementation, the scan pattern is stored in a memory (e.g. memory 704) of the processing system 250. In another example implementation, the angle increments between adjacent angles in the scan pattern is determined in step 615, e.g. the angle increment is the spacing between the subsequent angle and the initial angle in step 615. In another example implementation, FIG. 3C depicts an angle increment 350 a between a subsequent angle 345 b and an initial angle 345 a for purposes of step 615 and the scan pattern determined in step 617.

In step 619, the LIDAR system is operated according to the scan pattern determined in step 617. In one implementation, where the maximum design range has a fixed value or a fixed range of values over the angle range 227, the fixed maximum scan rate is determined in step 609, based on the fixed value or fixed range of values of the maximum design range. In this implementation, in step 619 the beam of the LIDAR system is scanned at the fixed maximum scan rate using the scanning optics 218 over the angle range 227. In an example implementation, in step 619 the beam 205″ is scanned using the polygon scanner 244 at the fixed maximum scan rate. In one implementation, the processing system 250 transmits a signal to the polygon scanner 244 to cause the polygon scanner 244 so that the speed of the angular velocity 249 is the fixed maximum scan rate. Additionally, in this implementation, the minimum integration time of the LIDAR system is based on the fixed minimum integration time based on the fixed value or fixed range of values for the maximum design range. In an implementation, the beam 205″ is scanned continuously over the angle range 227 as the beam 205″ is scanned by adjacent facets 245 of the polygon scanner 244. Thus, in an example implementation, the beam 205″ is scanned by the facet 245 a over the angle range 227 and the beam 205″ is subsequently scanned by the facet 245 b over the angle range 227. During step 619, the return beam 291′ is focused by the collimation optic 229 into the tip 217 of the receiving waveguide 225. In an example implementation, where multiple receiving waveguides 225 a, 225 b are provided, the return beams 291′ are focused into the tip 217 of the first receiving waveguide 225 a when the beam 205″ is scanned at the fixed maximum scan rate over a first portion of the angle range 227 (e.g. further target range; beam 344) and the return beams 291′ are focused into the tip of the second receiving waveguide 225 b when the beam 205″ is scanned at the fixed maximum scan rate over a second portion of the angle range 227 (e.g. shorter target range; beams 342, 346). In another example implementation, one receiving waveguide 225 a and the receiving waveguide 225 b is omitted and the return beams 291′ are received in the tip of the receiving waveguide 225 a as the beam 205″ is scanned at the fixed maximum scan rate over the angle range 227 (e.g. field of view 324).

In step 619, one or more parameters of the LIDAR system are selected during a design phase of the system 200″. In an example implementation, a value of the separation 221 between the transmission waveguide 223 and the receiving waveguide 225 is selected based on the use of one or more of the graphs 464, 465, 466, 467, 469 where the user determines a value of the separation 221 based on known values of the design target range and scan speed in order to achieve an SNR threshold of the return beam 291′. In yet another example implementation, a value of the scan speed of the polygon scanner 244 is selected based on the use of the graphs 464, 465, 466, 467, 469 and graphs 440, where the user determines a value of the scan speed based on known values of the design target range and separation 221 in order to achieve an SNR threshold of the return beam 291′.

In another implementation, in step 619 the beam of the LIDAR system is scanned in the angle range 326 over one or more cycles, where the scan rate of the beam at each angle is the maximum scan rate in the scan pattern for that angle and the integration time of the LIDAR system at each angle is the minimum integration time for that angle. In one implementation, in step 619, at each angle the processing system 250 of the LIDAR system transmits one or more signals to the scanning optics 218 so that the scan rate at each angle is the maximum scan rate of the scan pattern for that angle. Additionally, in one implementation, in step 619, at each angle the processing system 250 of the LIDAR system adjusts the integration time of the acquisition system 240 and/or processing system 250 for the return beam 291 received at each angle so that the integration time is the minimum integration time of the scan pattern for that angle. This advantageously ensures that the beam is scanned at the maximum scan rate and that the return beam is processed at the shortest integration time at each angle, while ensuring the LIDAR system maintains an adequate SNR at each angle.

FIG. 5 is a graph that illustrates an example of the vertical angle over time over multiple angle ranges in the system of FIG. 2E, according to an implementation. The horizontal axis 502 is time in units of seconds (s) and the vertical axis is angle in units of radians (rad). Curve 540 depicts the angle of the beam over time during the scanning over the beam during multiple scan patterns in step 619. A slope of the curve 540 at an instant in time indicates the scan rate of the beam at that time. In an implementation, region 542 of the curve 540 demonstrates a faster scan rate (e.g. high slope of the curve 540) when the beam is oriented towards the ceiling 347; region 544 of the curve 540 also demonstrates a faster scan rate (e.g. high slope of curve 540) when the beam is oriented towards the surface 349; and region 546 of the curve 540 demonstrates a slower scan rate (e.g. lower slope of the curve 540) when the beam is oriented about parallel with the surface 349. In other implementations, where the beam 205″ is scanned at the fixed maximum scan rate, the curve would demonstrate a fixed slope over the angle range 227.

In another implementation, during or after step 619, the processor operates the vehicle 310 based at least in part on the data collected by the LIDAR system during step 619. In one implementation, the processing system 250 of the LIDAR system and/or the processor 314 of the vehicle 310 transmit one or more signals to the steering and/or braking system of the vehicle based on the data collected by the LIDAR system in step 619. In one example implementation, the processing system 250 transmits one or more signals to the steering or braking system of the vehicle 310 to control a position of the vehicle 310 in response to the LIDAR data. In other implementations, the processing system 250 transmits one or more signals to the processor 314 of the vehicle 310 based on the LIDAR data collected in step 619 and the processor 314 in turn transmits one or more signals to the steering and braking system of the vehicle 310.

8. Computational Hardware Overview

FIG. 7 is a block diagram that illustrates a computer system 700 upon which an implementation of the disclosure may be implemented. Computer system 700 includes a communication mechanism such as a bus 710 for passing information between other internal and external components of the computer system 700. Information is represented as physical signals of a measurable phenomenon, typically electric voltages, but including, in other implementations, such phenomena as magnetic, electromagnetic, pressure, chemical, molecular atomic and quantum interactions. For example, north and south magnetic fields, or a zero and non-zero electric voltage, represent two states (0, 1) of a binary digit (bit). Other phenomena can represent digits of a higher base. A superposition of multiple simultaneous quantum states before measurement represents a quantum bit (qubit). A sequence of one or more digits constitutes digital data that is used to represent a number or code for a character. In some implementations, information called analog data is represented by a near continuum of measurable values within a particular range. Computer system 700, or a portion thereof, constitutes a means for performing one or more steps of one or more methods described herein.

A sequence of binary digits constitutes digital data that is used to represent a number or code for a character. A bus 710 includes many parallel conductors of information so that information is transferred quickly among devices coupled to the bus 710. One or more processors 702 for processing information are coupled with the bus 710. A processor 702 performs a set of operations on information. The set of operations include bringing information in from the bus 710 and placing information on the bus 710. The set of operations also typically include comparing two or more units of information, shifting positions of units of information, and combining two or more units of information, such as by addition or multiplication. A sequence of operations to be executed by the processor 702 constitutes computer instructions.

Computer system 700 also includes a memory 704 coupled to bus 710. The memory 704, such as a random access memory (RAM) or other dynamic storage device, stores information including computer instructions. Dynamic memory allows information stored therein to be changed by the computer system 700. RAM allows a unit of information stored at a location called a memory address to be stored and retrieved independently of information at neighboring addresses. The memory 704 is also used by the processor 702 to store temporary values during execution of computer instructions. The computer system 700 also includes a read only memory (ROM) 706 or other static storage device coupled to the bus 710 for storing static information, including instructions, that is not changed by the computer system 700. Also coupled to bus 710 is a non-volatile (persistent) storage device 708, such as a magnetic disk or optical disk, for storing information, including instructions, that persists even when the computer system 700 is turned off or otherwise loses power.

Information, including instructions, is provided to the bus 710 for use by the processor from an external input device 712, such as a keyboard containing alphanumeric keys operated by a human user, or a sensor. A sensor detects conditions in its vicinity and transforms those detections into signals compatible with the signals used to represent information in computer system 700. Other external devices coupled to bus 710, used primarily for interacting with humans, include a display device 714, such as a cathode ray tube (CRT) or a liquid crystal display (LCD), for presenting images, and a pointing device 716, such as a mouse or a trackball or cursor direction keys, for controlling a position of a small cursor image presented on the display 714 and issuing commands associated with graphical elements presented on the display 714.

In the illustrated implementation, special purpose hardware, such as an application specific integrated circuit (IC) 720, is coupled to bus 710. The special purpose hardware is configured to perform operations not performed by processor 702 quickly enough for special purposes. Examples of application specific ICs include graphics accelerator cards for generating images for display 714, cryptographic boards for encrypting and decrypting messages sent over a network, speech recognition, and interfaces to special external devices, such as robotic arms and medical scanning equipment that repeatedly perform some complex sequence of operations that are more efficiently implemented in hardware.

Computer system 700 also includes one or more instances of a communications interface 770 coupled to bus 710. Communication interface 770 provides a two-way communication coupling to a variety of external devices that operate with their own processors, such as printers, scanners and external disks. In general the coupling is with a network link 778 that is connected to a local network 780 to which a variety of external devices with their own processors are connected. For example, communication interface 770 may be a parallel port or a serial port or a universal serial bus (USB) port on a personal computer. In some implementations, communications interface 770 is an integrated services digital network (ISDN) card or a digital subscriber line (DSL) card or a telephone modem that provides an information communication connection to a corresponding type of telephone line. In some implementations, a communication interface 770 is a cable modem that converts signals on bus 710 into signals for a communication connection over a coaxial cable or into optical signals for a communication connection over a fiber optic cable. As another example, communications interface 770 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN, such as Ethernet. Wireless links may also be implemented. Carrier waves, such as acoustic waves and electromagnetic waves, including radio, optical and infrared waves travel through space without wires or cables. Signals include man-made variations in amplitude, frequency, phase, polarization or other physical properties of carrier waves. For wireless links, the communications interface 770 sends and receives electrical, acoustic or electromagnetic signals, including infrared and optical signals, that carry information streams, such as digital data.

The term computer-readable medium is used herein to refer to any medium that participates in providing information to processor 702, including instructions for execution. Such a medium may take many forms, including, but not limited to, non-volatile media, volatile media and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 708. Volatile media include, for example, dynamic memory 704. Transmission media include, for example, coaxial cables, copper wire, fiber optic cables, and waves that travel through space without wires or cables, such as acoustic waves and electromagnetic waves, including radio, optical and infrared waves. The term computer-readable storage medium is used herein to refer to any medium that participates in providing information to processor 702, except for transmission media.

Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, a hard disk, a magnetic tape, or any other magnetic medium, a compact disk ROM (CD-ROM), a digital video disk (DVD) or any other optical medium, punch cards, paper tape, or any other physical medium with patterns of holes, a RAM, a programmable ROM (PROM), an erasable PROM (EPROM), a FLASH-EPROM, or any other memory chip or cartridge, a carrier wave, or any other medium from which a computer can read. The term non-transitory computer-readable storage medium is used herein to refer to any medium that participates in providing information to processor 702, except for carrier waves and other signals.

Logic encoded in one or more tangible media includes one or both of processor instructions on a computer-readable storage media and special purpose hardware, such as ASIC 720.

Network link 778 typically provides information communication through one or more networks to other devices that use or process the information. For example, network link 778 may provide a connection through local network 780 to a host computer 782 or to equipment 784 operated by an Internet Service Provider (ISP). ISP equipment 784 in turn provides data communication services through the public, world-wide packet-switching communication network of networks now commonly referred to as the Internet 790. A computer called a server 792 connected to the Internet provides a service in response to information received over the Internet. For example, server 792 provides information representing video data for presentation at display 714.

The disclosure is related to the use of computer system 700 for implementing the techniques described herein. According to one implementation of the disclosure, those techniques are performed by computer system 700 in response to processor 702 executing one or more sequences of one or more instructions contained in memory 704. Such instructions, also called software and program code, may be read into memory 704 from another computer-readable medium such as storage device 708. Execution of the sequences of instructions contained in memory 704 causes processor 702 to perform the method steps described herein. In alternative implementations, hardware, such as application specific integrated circuit 720, may be used in place of or in combination with software to implement the disclosure. Thus, implementations of the disclosure are not limited to any specific combination of hardware and software.

The signals transmitted over network link 778 and other networks through communications interface 770, carry information to and from computer system 700. Computer system 700 can send and receive information, including program code, through the networks 780, 790 among others, through network link 778 and communications interface 770. In an example using the Internet 790, a server 792 transmits program code for a particular application, requested by a message sent from computer 700, through Internet 790, ISP equipment 784, local network 780 and communications interface 770. The received code may be executed by processor 702 as it is received, or may be stored in storage device 708 or other non-volatile storage for later execution, or both. In this manner, computer system 700 may obtain application program code in the form of a signal on a carrier wave.

Various forms of computer readable media may be involved in carrying one or more sequence of instructions or data or both to processor 702 for execution. For example, instructions and data may initially be carried on a magnetic disk of a remote computer such as host 782. The remote computer loads the instructions and data into its dynamic memory and sends the instructions and data over a telephone line using a modem. A modem local to the computer system 700 receives the instructions and data on a telephone line and uses an infra-red transmitter to convert the instructions and data to a signal on an infra-red a carrier wave serving as the network link 778. An infrared detector serving as communications interface 770 receives the instructions and data carried in the infrared signal and places information representing the instructions and data onto bus 710. Bus 710 carries the information to memory 704 from which processor 702 retrieves and executes the instructions using some of the data sent with the instructions. The instructions and data received in memory 704 may optionally be stored on storage device 708, either before or after execution by the processor 702.

FIG. 8 illustrates a chip set 800 upon which an embodiment of the disclosure may be implemented. Chip set 800 is programmed to perform one or more steps of a method described herein and includes, for instance, the processor and memory components described with respect to FIG. 7 incorporated in one or more physical packages (e.g., chips). By way of example, a physical package includes an arrangement of one or more materials, components, and/or wires on a structural assembly (e.g., a baseboard) to provide one or more characteristics such as physical strength, conservation of size, and/or limitation of electrical interaction. It is contemplated that in certain embodiments the chip set can be implemented in a single chip. Chip set 800, or a portion thereof, constitutes a means for performing one or more steps of a method described herein.

In one implementation, the chip set 800 includes a communication mechanism such as a bus 801 for passing information among the components of the chip set 800. A processor 803 has connectivity to the bus 801 to execute instructions and process information stored in, for example, a memory 805. The processor 803 may include one or more processing cores with each core configured to perform independently. A multi-core processor enables multiprocessing within a single physical package. Examples of a multi-core processor include two, four, eight, or greater numbers of processing cores. Alternatively or in addition, the processor 803 may include one or more microprocessors configured in tandem via the bus 801 to enable independent execution of instructions, pipelining, and multithreading. The processor 803 may also be accompanied with one or more specialized components to perform certain processing functions and tasks such as one or more digital signal processors (DSP) 807, or one or more application-specific integrated circuits (ASIC) 809. A DSP 807 typically is configured to process real-world signals (e.g., sound) in real time independently of the processor 803. Similarly, an ASIC 809 can be configured to performed specialized functions not easily performed by a general purposed processor. Other specialized components to aid in performing the inventive functions described herein include one or more field programmable gate arrays (FPGA) (not shown), one or more controllers (not shown), or one or more other special-purpose computer chips.

The processor 803 and accompanying components have connectivity to the memory 805 via the bus 801. The memory 805 includes both dynamic memory (e.g., RAM, magnetic disk, writable optical disk, etc.) and static memory (e.g., ROM, CD-ROM, etc.) for storing executable instructions that when executed perform one or more steps of a method described herein. The memory 805 also stores the data associated with or generated by the execution of one or more steps of the methods described herein.

9. Alterations, Extensions and Modifications

In the foregoing specification, the disclosure has been described with reference to specific implementations thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the disclosure. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. Throughout this specification and the claims, unless the context requires otherwise, the word “comprise” and its variations, such as “comprises” and “comprising,” will be understood to imply the inclusion of a stated item, element or step or group of items, elements or steps but not the exclusion of any other item, element or step or group of items, elements or steps. Furthermore, the indefinite article “a” or “an” is meant to indicate one or more of the item, element or step modified by the article. As used herein, unless otherwise clear from the context, a value is “about” another value if it is within a factor of two (twice or half) of the other value. While example ranges are given, unless otherwise clear from the context, any contained ranges are also intended in various implementations. Thus, a range from 0 to 10 includes the range 1 to 4 in some implementations.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope are approximations, the numerical values set forth in specific non-limiting examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements at the time of this writing. Furthermore, unless otherwise clear from the context, a numerical value presented herein has an implied precision given by the least significant digit. Thus a value 1.1 implies a value from 1.05 to 1.15. The term “about” is used to indicate a broader range centered on the given value, and unless otherwise clear from the context implies a broader range around the least significant digit, such as “about 1.1” implies a range from 1.0 to 1.2. If the least significant digit is unclear, then the term “about” implies a factor of two, e.g., “about X” implies a value in the range from 0.5× to 2×, for example, about 100 implies a value in a range from 50 to 200. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of “less than 10” can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 4.

Some implementations of the disclosure are described below in the context of binary, π/2 (90 degree) phase encoding at a radio frequency modulated onto an optical signal; but, implementations are not limited to this context. In other implementations, other phase encoding is used, with different phase differences (e.g., 30, 60, or 180 degrees) or encoding with 3 or more different phases. Implementations are described in the context of a single optical beam and its return on a single detector or pair of detectors, which in other implementations can then be scanned using any known scanning means, such as linear stepping or rotating optical components or with arrays of transmitters or arrays of detectors or pairs of detectors. For purposes of this description, “phase code duration” is a duration of a code that indicates a sequence of phases for a phase-encoded signal modulated onto an optical signal.

In some implementations, an apparatus includes a bistatic transceiver, a birefringent displacer, a polarization transforming optic, and a collimation optic. The bistatic transceiver may include a transmission waveguide configured to transmit a first signal from a laser source, and a receiving waveguide spaced apart from the transmission waveguide by a separation and configured to receive a second signal reflected by a target. The birefringent displacer may be configured to displace one of the first signal and the second signal in a direction orthogonal to a direction of the one of the first signal and the second signal. The polarization transforming optic may be configured to adjust a relative phase between orthogonal field components of the first signal and the second signal. The collimation optic may be configured to shape the first signal transmitted from the transmission waveguide and to shape the second signal reflected by the target. The birefringent displacer and the polarization transforming optic are positioned between the bistatic transceiver and the collimation optic.

In some implementations, the birefringent displacer may be configured to displace the one of the first signal and second signal by a distance orthogonal to the direction of the one of the first signal and second signal, and the distance is based on the separation.

In some implementations, the birefringent displacer may have a dimension along a longitudinal axis of the apparatus, and the dimension is sized such that the distance is about equal to the separation.

In some implementations, the bistatic transceiver may include one or more pairs of waveguides. Each pair may include one transmission waveguide and one receiving waveguide. The birefringent displacer may be configured so that the first signal is not displaced by the birefringent displacer and is further configured to displace the second signal based on the separation so that the displaced second signal is incident on the receiving waveguide.

In some implementations, the transmission waveguide and the receiving waveguide may be polarization maintaining fibers with orthogonal orientations. The first signal and second signal may be linearly polarized with orthogonal polarization based on the orthogonal orientation of the polarization maintaining fibers.

In some implementations, the polarization transforming optic may be a quarter wave plate. The quarter wave plate may be configured to adjust a first linear polarization of the first signal into a first circular polarization in a first direction. The quarter wave plate may be further configured to adjust a second circular polarization of the second signal reflected by the target into a second linear polarization orthogonal to the first linear polarization. The second circular polarization may have a second direction opposite to the first direction.

In some implementations, the bistatic transceiver may include one transmission waveguide and a pair of receiving waveguides including a first receiving waveguide spaced apart from a first side of the transmission waveguide by a first separation and a second receiving waveguide spaced apart from a second side of the transmission waveguide by a second separation. The transmission waveguide may include a single mode fiber, and the first and second receiving waveguides may be polarization maintaining fibers with orthogonal orientation. The first signal may be nonpolarized. The first and second receiving waveguides may be configured to receive respective second signals that are linearly polarized with orthogonal polarization based on the orthogonal orientation of the polarization maintaining fibers.

In some implementations, the birefringent displacer may include a first birefringent displacer and a second birefringent displacer with the polarization transforming optic positioned between the first and second birefringent displacers. The first birefringent displacer may be configured to displace a first component of the first signal based on a first linear polarization of the first component and may be further configured to not displace a second component of the first signal based on a second linear polarization of the second component orthogonal to the first linear polarization. The polarization transforming optic may be configured to rotate the first linear polarization of the first component to the second linear polarization and may be further configured to rotate the second linear polarization of the second component to the first linear polarization. The second birefringent displacer may be configured to displace the second component of the first signal incident from the polarization transforming optic based on the first linear polarization of the second component and may be further configured to not displace the first component of the first signal based on the second linear polarization of the first component. The first signal incident on the first birefringent displacer may be non polarized. The first birefringent displacer may be configured to displace the first component of the first signal based on the first separation. The second birefringent displacer may be configured to displace the second component of the first signal based on the first separation. The second birefringent displacer may be configured to displace a first component of the second signal reflect by the target based on a first linear polarization of the first component and may be further configured to not displace a second component of the second signal based on a second linear polarization of the second component orthogonal to the first linear polarization. The first birefringent displacer may be configured to displace the first component of the second signal based on the first linear polarization of the first component and may be further configured to not displace the second component of the second signal based on the second linear polarization of the first component. The second signal from the target incident on the second birefringent displacer may be non polarized. The second birefringent displacer may be configured to displace the first component of the second signal based on the first separation. The first birefringent displacer may be configured to displace the first component of the second signal based on the second separation.

In some implementations, the polarization transforming optic may be configured to not affect the linear polarization of the first and second components of the second signal incident on the polarization transforming optic from the second birefringent displacer. The polarization optic may include a faraday rotator and half wave plate. The half wave plate may be configured to rotate the linear polarization of the first and second components of the second signal. The faraday rotator may be configured to rotate the linear polarization of the first and second components of the second components by an amount equal and opposite to the half wave plate.

In some implementations, the apparatus may include scanning optics configured to adjust a direction of the first signal in a scan direction at a scan rate over an angle range defined by a first angle and a second angle. The scan direction may be non parallel with respect to a direction of a vector from the transmission waveguide to the receiving waveguide. The birefringent displacer may be oriented at an angle relative to the vector such that the displaced second signal is received in the receiving waveguide. A direction of the angle with respect to the vector may be based on the scan direction and a magnitude of the angle is based on the scan rate.

In some implementations, the scan direction may be about parallel with respect to a direction of a vector from the transmission waveguide to the receiving waveguide. The birefringent displacer may be configured so that a magnitude of the displacement of the second signal is based on the scan direction. A dimension of the birefringent displacer along a longitudinal axis of the apparatus may be selected to adjust the magnitude of the displacement. In some implementations, the scan direction may be in a same direction as the vector. The magnitude of the displacement may be greater than a magnitude of the vector such that the displaced second signal is received in the receiving waveguide.

In some implementations, a method includes transmitting, from a laser source, a first signal from a transmission waveguide of a bistatic transceiver. The method also includes receiving, at a receiving waveguide of the bistatic transceiver, a second signal reflected by a target. The receiving waveguide may be spaced apart from the transmission waveguide by a separation. The method also includes displacing, with a birefringent displacer, one of the first signal and the second signal in a direction orthogonal to a direction of the one of the first signal and the second signal. The method also includes adjusting, with a polarization transforming optic, a relative phase between orthogonal field components of the first signal and the second signal. The method also includes shaping, with a collimation optic, the first signal transmitted from the transmission waveguide and the second signal reflected from the target. The birefringent displacer and the polarization optic may be positioned between the bistatic transceiver and the collimation optic.

In some implementations, the displacing may include displacing the one of the first signal and the second signal by a distance orthogonal to the direction of the one of the first signal and second signal. The distance may be based on the separation.

In some implementations, the displacing may include not displacing, with the birefringent displacer, the first signal in the direction orthogonal to the direction of the first signal and displacing the second signal in the direction orthogonal to the direction of the second signal based on the separation so that the displaced second signal is incident on the receiving waveguide.

In some implementations, the polarization transforming optic may be a quarter wave plate. The adjusting step may include adjusting, with the quarter wave plate, a first linear polarization of the first signal into a first circular polarization in a first direction. The adjusting step may also include adjusting, with the quarter wave plate, a second circular polarization of the second signal reflected by the target into a second linear polarization orthogonal to the first linear polarization. The second circular polarization may have a second direction opposite to the first direction.

In some implementations, the birefringent displacer may include a first birefringent displacer and a second birefringent displacer with the polarization transforming optic positioned between the first and second birefringent displacers. The displacing, with the first birefringent displacer may include displacing a first component of the first signal based on a first linear polarization of the first component and not displacing a second component of the first signal based on a second linear polarization of the second component orthogonal to the first linear polarization. The adjusting, with the polarization transforming optic, may include rotating the first linear polarization of the first component to the second linear polarization and rotating the second linear polarization of the second component to the first linear polarization/The displacing, with the second birefringent displacer, may include displacing the second component of the first signal incident from the polarization transforming optic based on the first linear polarization of the second component and not displacing the first component of the first signal based on the second linear polarization of the first component.

In some implementations, the bistatic transceiver may include one transmission waveguide and a pair of receiving waveguides including a first receiving waveguide spaced apart from a first side of the transmission waveguide by a first separation and a second receiving waveguide spaced apart from a second side of the transmission waveguide by a second separation. The first signal incident on the first birefringent displacer may be non polarized. The displacing of the first component of the first signal with the first birefringent displacer may be based on the first separation. The displacing of the second component of the first signal with the second birefringent displacer may be based the first separation.

In some implementations, the displacing, with the second birefringent displacer, may include displacing a first component of the second signal reflected by the target based on a first linear polarization of the first component and not displacing a second component of the second signal based on a second linear polarization of the second component orthogonal to the first linear polarization. The displacing, with the first birefringent displacer, may include displacing the first component of the second signal based on the first linear polarization of the first component and not displacing the second component of the second signal based on the second linear polarization of the first component.

In some implementations, the bistatic transceiver may include one transmission waveguide and a pair of receiving waveguides including a first receiving waveguide spaced apart from a first side of the transmission waveguide by a first separation and a second receiving waveguide spaced apart from a second side of the transmission waveguide by a second separation. The second signal from the target incident on the second birefringent displacer may be non polarized. The displacing of the first component of the second signal with the second birefringent displacer may be based on the first separation. The displacing of the first component of the second signal with the first birefringent displacer may be based on the second separation.

In some implementations, the polarization transforming optic may be configured to not affect the linear polarization of the first and second components of the second signal incident on the polarization transforming optic from the second birefringent displacer. The polarization optic may include a faraday rotator and half wave plate. The adjusting step may include rotating, with the half wave plate, the linear polarization of the first and second components of the second signal, and rotating, with the faraday rotator, the linear polarization of the first and second components of the second signal by an amount equal and opposite to the half wave plate.

In some implementations, the method may include adjusting a direction of the first signal, with scanning optics, in a scan direction at a scan rate over an angle range defined by a first angle and a second angle. The scan direction may be nonparallel to a vector from the transmission waveguide to the receiving waveguide. The displacing may include orienting the birefringent displacer at an angle relative to the vector. The direction of the angle is based on the scan direction and a magnitude of the angle is based on the scan rate. The scan direction may be about parallel to a vector from the transmission waveguide to the receiving waveguide. The displacing may include adjusting a magnitude of the displacement based on the scan direction.

In some implementations, a system for optimizing a scan pattern of a LIDAR system, may include the LIDAR system including the above-described apparatus and the laser source. The system may further include a processor and at least one memory including one or more sequences of instructions. The one or more sequences of instructions, when executed by the processor, cause the system to transmit the first signal from the transmission waveguide, receive the second signal at the receiving waveguide, combine the second signal with a reference beam from the laser source in one or more optical mixers, generate a signal based on the combining of the second signal and the reference beam, and operate a device based on the signal. The apparatus may include a one-dimensional bistatic transceiver including the transmission waveguide and a plurality of receiving waveguides. The system further may include a plurality of processing channels. The step of receiving the second signal may include receiving a plurality of second signals at the respective plurality of receiving waveguides. The step of combining may include combining the plurality of second signals with the reference beam in the plurality of processing channels. Each processing channel may be assigned to a respective receiving waveguide. 

What is claimed is:
 1. A light detection and ranging (LIDAR) system comprising: a laser source that is configured to generate a beam; a transceiver configured to generate a transmit signal based on the beam and transmit the transmit signal through a transmission waveguide and to receive a return signal reflected by an object through a receiving waveguide; and one or more optics external to the transceiver and configured to optically change a distance between the transmit signal and the return signal by displacing one signal of the transmit signal or the return signal wherein the one signal before the displacement and the one signal after the displacement are parallel to each other.
 2. The LIDAR system as recited in claim 1, wherein the one or more optics are configured to displace the return signal in a first direction that is orthogonal to a second direction in which the return signal travels.
 3. The LIDAR system as recited in claim 2, wherein the one or more optics include a displacer having at least two refractive indexes, and the displacer is configured to displace the return signal in the first direction.
 4. The LIDAR system as recited in claim 3, further comprising a first optic configured to collimate the transmit signal transmitted from the transmission waveguide and to focus the return signal reflected by the object.
 5. The LIDAR system as recited in claim 4, further comprising: a polarization transforming optic configured to adjust polarizations of the transmit signal and the return signal into adjusted polarizations of the transmit signal and the return signal such that the adjusted polarization of the transmit signal is orthogonal to the adjusted polarization of the return signal.
 6. The LIDAR system as recited in claim 5, wherein the displacer and the polarization transforming optic are positioned between the transceiver and the first optic.
 7. The LIDAR system as recited in claim 3, wherein: the receiving waveguide is spaced apart from the transmission waveguide by a separation; the displacer is configured to displace the return signal by a first distance in the first direction; and the first distance is based on the separation.
 8. The LIDAR system as recited in claim 1, wherein the one or more optics are configured to displace the transmit signal in a third direction that is orthogonal to a fourth direction in which the transmit signal travels.
 9. An autonomous vehicle control system comprising one or more processors, wherein the one or more processors are configured to: cause a laser source to generate a beam; cause a transceiver to generate a transmit signal based on the beam and transmit the transmit signal through a transmission waveguide and to receive a return signal reflected by an object through a receiving waveguide; cause one or more optics to optically change a distance between the transmit signal and the return signal by displacing one signal of the transmit signal or the return signal wherein the one signal before the displacement and the one signal after the displacement are parallel to each other; and operate a vehicle based on the return signal received by the transceiver.
 10. The autonomous vehicle control system as recited in claim 9, wherein: the one or more optics includes a displacer having at least two refractive indexes; and the one or more processors are configured to cause the displacer to displace the return signal in a first direction that is orthogonal to a second direction in which the return signal travels.
 11. The autonomous vehicle control system as recited in claim 10, wherein the one or more processors are configured to cause a first optic to collimate the transmit signal transmitted from the transmission waveguide and to focus the return signal reflected by the object.
 12. The autonomous vehicle control system as recited in claim 11, wherein: the one or more processors are configured to cause a polarization transforming optic to adjust polarizations of the transmit signal and the return signal into adjusted polarizations of the transmit signal and the return signal such that the adjusted polarization of the transmit signal is orthogonal to the adjusted polarization of the return signal; and the displacer and the polarization transforming optic are positioned between the transceiver and the first optic.
 13. The autonomous vehicle control system as recited in claim 12, wherein: the receiving waveguide is spaced apart from the transmission waveguide by a separation; the one or more processors are configured to cause the displacer to displace the return signal by a first distance in the first direction; and the first distance is based on the separation.
 14. The autonomous vehicle control system as recited in claim 10, wherein the one or more processors are configured to cause the displacer to displace the transmit signal in a third direction that is orthogonal to a fourth direction in which the transmit signal travels.
 15. An autonomous vehicle comprising a light detection and ranging (LIDAR) system, wherein the LIDAR system comprises: a laser source that is configured to generated a beam; a transceiver configured to generate a transmit signal based on the beam and transmit the transmit signal through a transmission waveguide and to receive a return signal reflected by an object through a receiving waveguide; and one or more optics external to the transceiver and configured to optically change a distance between the transmit signal and the return signal by displacing one signal of the transmit signal or the return signal wherein the one signal before the displacement and the one signal after the displacement are parallel to each other.
 16. The autonomous vehicle as recited in claim 15, wherein the one or more optics include a displacer having at least two refractive indexes, and the displacer is configured to displace the return signal in a first direction that is orthogonal to a second direction in which the return signal travels.
 17. The autonomous vehicle as recited in claim 16, wherein the LIDAR system further comprises a first optic configured to collimate the transmit signal transmitted from the transmission waveguide and to focus the return signal reflected by the object.
 18. The autonomous vehicle as recited in claim 17, wherein the LIDAR system further comprises: a polarization transforming optic configured to adjust polarizations of the transmit signal and the return signal in to adjusted polarizations of the transmit signal and the return signal such that the adjusted polarization of the transmit signal is orthogonal to the adjusted polarization of the return signal, wherein the displacer and the polarization transforming optic are positioned between the transceiver and the collimation optic.
 19. The autonomous vehicle as recited in claim 18, wherein: the receiving waveguide is spaced apart from the transmission waveguide by a separation; the displacer is configured to displace the return signal by a first distance in the first direction; and the first distance is based on the separation.
 20. The autonomous vehicle as recited in claim 16, wherein the LIDAR system the displacer is configured to displace the transmit signal in a third direction that is orthogonal to a fourth direction in which the transmit signal travels. 